NFS: Fix a race when doing NFS write coalescing

Currently we do write coalescing in a very inefficient manner: one pass in
generic_writepages() in order to lock the pages for writing, then one pass
in nfs_flush_mapping() and/or nfs_sync_mapping_wait() in order to gather
the locked pages for coalescing into RPC requests of size "wsize".

In fact, it turns out there is actually a deadlock possible here since we
only start I/O on the second pass. If the user signals the process while
we're in nfs_sync_mapping_wait(), for instance, then we may exit before
starting I/O on all the requests that have been queued up.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

2 files changed, 47 insertions, 194 deletions

diff --git a/fs/nfs/pagelist.c b/fs/nfs/pagelist.c
index 094537ddd344..ea1a85df9ab1 100644
--- a/fs/nfs/pagelist.c
+++ b/fs/nfs/pagelist.c
@@ -17,7 +17,6 @@
 #include <linux/nfs_page.h>
 #include <linux/nfs_fs.h>
 #include <linux/nfs_mount.h>
-#include <linux/writeback.h>
 
 #define NFS_PARANOIA 1
 
@@ -354,25 +353,6 @@ int nfs_pageio_add_request(struct nfs_pageio_descriptor *desc,
 }
 
 /**
- * nfs_pageio_add_list - Split coalesced requests out from a list.
- * @desc: destination io descriptor
- * @head: source list
- *
- * Moves a maximum of 'nmax' elements from one list to another.
- * The elements are checked to ensure that they form a contiguous set
- * of pages, and that the RPC credentials are the same.
- */
-void nfs_pageio_add_list(struct nfs_pageio_descriptor *desc,
-                         struct list_head *head)
-{
-        while (!list_empty(head)) {
-                struct nfs_page *req = nfs_list_entry(head->next);
-                if (!nfs_pageio_add_request(desc, req))
-                        break;
-        }
-}
-
-/**
  * nfs_pageio_complete - Complete I/O on an nfs_pageio_descriptor
  * @desc: pointer to io descriptor
  */
@@ -383,78 +363,6 @@ void nfs_pageio_complete(struct nfs_pageio_descriptor *desc)
 
 #define NFS_SCAN_MAXENTRIES 16
 /**
- * nfs_scan_dirty - Scan the radix tree for dirty requests
- * @mapping: pointer to address space
- * @wbc: writeback_control structure
- * @dst: Destination list
- *
- * Moves elements from one of the inode request lists.
- * If the number of requests is set to 0, the entire address_space
- * starting at index idx_start, is scanned.
- * The requests are *not* checked to ensure that they form a contiguous set.
- * You must be holding the inode's req_lock when calling this function
- */
-long nfs_scan_dirty(struct address_space *mapping,
-                        struct writeback_control *wbc,
-                        struct list_head *dst)
-{
-        struct nfs_inode *nfsi = NFS_I(mapping->host);
-        struct nfs_page *pgvec[NFS_SCAN_MAXENTRIES];
-        struct nfs_page *req;
-        pgoff_t idx_start, idx_end;
-        long res = 0;
-        int found, i;
-
-        if (nfsi->ndirty == 0)
-                return 0;
-        if (wbc->range_cyclic) {
-                idx_start = 0;
-                idx_end = ULONG_MAX;
-        } else if (wbc->range_end == 0) {
-                idx_start = wbc->range_start >> PAGE_CACHE_SHIFT;
-                idx_end = ULONG_MAX;
-        } else {
-                idx_start = wbc->range_start >> PAGE_CACHE_SHIFT;
-                idx_end = wbc->range_end >> PAGE_CACHE_SHIFT;
-        }
-
-        for (;;) {
-                unsigned int toscan = NFS_SCAN_MAXENTRIES;
-
-                found = radix_tree_gang_lookup_tag(&nfsi->nfs_page_tree,
-                                (void **)&pgvec[0], idx_start, toscan,
-                                NFS_PAGE_TAG_DIRTY);
-
-                /* Did we make progress? */
-                if (found <= 0)
-                        break;
-
-                for (i = 0; i < found; i++) {
-                        req = pgvec[i];
-                        if (!wbc->range_cyclic && req->wb_index > idx_end)
-                                goto out;
-
-                        /* Try to lock request and mark it for writeback */
-                        if (!nfs_set_page_writeback_locked(req))
-                                goto next;
-                        radix_tree_tag_clear(&nfsi->nfs_page_tree,
-                                        req->wb_index, NFS_PAGE_TAG_DIRTY);
-                        nfsi->ndirty--;
-                        nfs_list_remove_request(req);
-                        nfs_list_add_request(req, dst);
-                        res++;
-                        if (res == LONG_MAX)
-                                goto out;
-next:
-                        idx_start = req->wb_index + 1;
-                }
-        }
-out:
-        WARN_ON ((nfsi->ndirty == 0) != list_empty(&nfsi->dirty));
-        return res;
-}
-
-/**
  * nfs_scan_list - Scan a list for matching requests
  * @nfsi: NFS inode
  * @head: One of the NFS inode request lists
diff --git a/fs/nfs/write.c b/fs/nfs/write.c
index b6749967eead..6ce2d94e7b3f 100644
--- a/fs/nfs/write.c
+++ b/fs/nfs/write.c
@@ -38,7 +38,8 @@
 static struct nfs_page * nfs_update_request(struct nfs_open_context*,
                                             struct page *,
                                             unsigned int, unsigned int);
-static long nfs_flush_mapping(struct address_space *mapping, struct writeback_control *wbc, int how);
+static void nfs_pageio_init_write(struct nfs_pageio_descriptor *desc,
+                                  struct inode *inode, int ioflags);
 static const struct rpc_call_ops nfs_write_partial_ops;
 static const struct rpc_call_ops nfs_write_full_ops;
 static const struct rpc_call_ops nfs_commit_ops;
@@ -201,7 +202,7 @@ static int nfs_writepage_setup(struct nfs_open_context *ctx, struct page *page,
 static int wb_priority(struct writeback_control *wbc)
 {
         if (wbc->for_reclaim)
-                return FLUSH_HIGHPRI;
+                return FLUSH_HIGHPRI | FLUSH_STABLE;
         if (wbc->for_kupdate)
                 return FLUSH_LOWPRI;
         return 0;
@@ -251,7 +252,8 @@ static void nfs_end_page_writeback(struct page *page)
  * was not tagged.
  * May also return an error if the user signalled nfs_wait_on_request().
  */
-static int nfs_page_mark_flush(struct page *page)
+static int nfs_page_async_flush(struct nfs_pageio_descriptor *pgio,
+                                struct page *page)
 {
         struct nfs_page *req;
         struct nfs_inode *nfsi = NFS_I(page->mapping->host);
@@ -273,6 +275,8 @@ static int nfs_page_mark_flush(struct page *page)
                  *       request as dirty (in which case we don't care).
                  */
                 spin_unlock(req_lock);
+                /* Prevent deadlock! */
+                nfs_pageio_complete(pgio);
                 ret = nfs_wait_on_request(req);
                 nfs_release_request(req);
                 if (ret != 0)
@@ -283,21 +287,18 @@ static int nfs_page_mark_flush(struct page *page)
                 /* This request is marked for commit */
                 spin_unlock(req_lock);
                 nfs_unlock_request(req);
+                nfs_pageio_complete(pgio);
                 return 1;
         }
-        if (nfs_set_page_writeback(page) == 0) {
-                nfs_list_remove_request(req);
-                /* add the request to the inode's dirty list. */
-                radix_tree_tag_set(&nfsi->nfs_page_tree,
-                                req->wb_index, NFS_PAGE_TAG_DIRTY);
-                nfs_list_add_request(req, &nfsi->dirty);
-                nfsi->ndirty++;
-                spin_unlock(req_lock);
-                __mark_inode_dirty(page->mapping->host, I_DIRTY_PAGES);
-        } else
+        if (nfs_set_page_writeback(page) != 0) {
                 spin_unlock(req_lock);
+                BUG();
+        }
+        radix_tree_tag_set(&nfsi->nfs_page_tree, req->wb_index,
+                        NFS_PAGE_TAG_WRITEBACK);
         ret = test_bit(PG_NEED_FLUSH, &req->wb_flags);
-        nfs_unlock_request(req);
+        spin_unlock(req_lock);
+        nfs_pageio_add_request(pgio, req);
         return ret;
 }
 
@@ -306,6 +307,7 @@ static int nfs_page_mark_flush(struct page *page)
  */
 static int nfs_writepage_locked(struct page *page, struct writeback_control *wbc)
 {
+        struct nfs_pageio_descriptor mypgio, *pgio;
         struct nfs_open_context *ctx;
         struct inode *inode = page->mapping->host;
         unsigned offset;
@@ -314,7 +316,14 @@ static int nfs_writepage_locked(struct page *page, struct writeback_control *wbc
         nfs_inc_stats(inode, NFSIOS_VFSWRITEPAGE);
         nfs_add_stats(inode, NFSIOS_WRITEPAGES, 1);
 
-        err = nfs_page_mark_flush(page);
+        if (wbc->for_writepages)
+                pgio = wbc->fs_private;
+        else {
+                nfs_pageio_init_write(&mypgio, inode, wb_priority(wbc));
+                pgio = &mypgio;
+        }
+
+        err = nfs_page_async_flush(pgio, page);
         if (err <= 0)
                 goto out;
         err = 0;
@@ -331,12 +340,12 @@ static int nfs_writepage_locked(struct page *page, struct writeback_control *wbc
         put_nfs_open_context(ctx);
         if (err != 0)
                 goto out;
-        err = nfs_page_mark_flush(page);
+        err = nfs_page_async_flush(pgio, page);
         if (err > 0)
                 err = 0;
 out:
         if (!wbc->for_writepages)
-                nfs_flush_mapping(page->mapping, wbc, FLUSH_STABLE|wb_priority(wbc));
+                nfs_pageio_complete(pgio);
         return err;
 }
 
@@ -352,20 +361,20 @@ int nfs_writepage(struct page *page, struct writeback_control *wbc)
 int nfs_writepages(struct address_space *mapping, struct writeback_control *wbc)
 {
         struct inode *inode = mapping->host;
+        struct nfs_pageio_descriptor pgio;
         int err;
 
         nfs_inc_stats(inode, NFSIOS_VFSWRITEPAGES);
 
+        nfs_pageio_init_write(&pgio, inode, wb_priority(wbc));
+        wbc->fs_private = &pgio;
         err = generic_writepages(mapping, wbc);
+        nfs_pageio_complete(&pgio);
         if (err)
                 return err;
-        err = nfs_flush_mapping(mapping, wbc, wb_priority(wbc));
-        if (err < 0)
-                goto out;
-        nfs_add_stats(inode, NFSIOS_WRITEPAGES, err);
-        err = 0;
-out:
-        return err;
+        if (pgio.pg_error)
+                return pgio.pg_error;
+        return 0;
 }
 
 /*
@@ -536,18 +545,6 @@ static int nfs_wait_on_requests_locked(struct inode *inode, unsigned long idx_st
         return res;
 }
 
-static void nfs_cancel_dirty_list(struct list_head *head)
-{
-        struct nfs_page *req;
-        while(!list_empty(head)) {
-                req = nfs_list_entry(head->next);
-                nfs_list_remove_request(req);
-                nfs_end_page_writeback(req->wb_page);
-                nfs_inode_remove_request(req);
-                nfs_clear_page_writeback(req);
-        }
-}
-
 static void nfs_cancel_commit_list(struct list_head *head)
 {
         struct nfs_page *req;
@@ -936,33 +933,15 @@ static int nfs_flush_one(struct inode *inode, struct list_head *head, size_t cou
         return -ENOMEM;
 }
 
-static int nfs_flush_list(struct inode *inode, struct list_head *head, int npages, int how)
+static void nfs_pageio_init_write(struct nfs_pageio_descriptor *pgio,
+                                  struct inode *inode, int ioflags)
 {
-        struct nfs_pageio_descriptor desc;
-        int wpages = NFS_SERVER(inode)->wpages;
         int wsize = NFS_SERVER(inode)->wsize;
 
-        /* For single writes, FLUSH_STABLE is more efficient */
-        if (npages <= wpages && npages == NFS_I(inode)->npages
-                        && nfs_list_entry(head->next)->wb_bytes <= wsize)
-                how |= FLUSH_STABLE;
-
         if (wsize < PAGE_CACHE_SIZE)
-                nfs_pageio_init(&desc, inode, nfs_flush_multi, wsize, how);
+                nfs_pageio_init(pgio, inode, nfs_flush_multi, wsize, ioflags);
         else
-                nfs_pageio_init(&desc, inode, nfs_flush_one, wsize, how);
-        nfs_pageio_add_list(&desc, head);
-        nfs_pageio_complete(&desc);
-        if (desc.pg_error == 0)
-                return 0;
-        while (!list_empty(head)) {
-                struct nfs_page *req = nfs_list_entry(head->next);
-                nfs_list_remove_request(req);
-                nfs_redirty_request(req);
-                nfs_end_page_writeback(req->wb_page);
-                nfs_clear_page_writeback(req);
-        }
-        return desc.pg_error;
+                nfs_pageio_init(pgio, inode, nfs_flush_one, wsize, ioflags);
 }
 
 /*
@@ -1286,31 +1265,7 @@ static const struct rpc_call_ops nfs_commit_ops = {
         .rpc_call_done = nfs_commit_done,
         .rpc_release = nfs_commit_release,
 };
-#else
-static inline int nfs_commit_list(struct inode *inode, struct list_head *head, int how)
-{
-        return 0;
-}
-#endif
-
-static long nfs_flush_mapping(struct address_space *mapping, struct writeback_control *wbc, int how)
-{
-        struct nfs_inode *nfsi = NFS_I(mapping->host);
-        LIST_HEAD(head);
-        long res;
-
-        spin_lock(&nfsi->req_lock);
-        res = nfs_scan_dirty(mapping, wbc, &head);
-        spin_unlock(&nfsi->req_lock);
-        if (res) {
-                int error = nfs_flush_list(mapping->host, &head, res, how);
-                if (error < 0)
-                        return error;
-        }
-        return res;
-}
 
-#if defined(CONFIG_NFS_V3) || defined(CONFIG_NFS_V4)
 int nfs_commit_inode(struct inode *inode, int how)
 {
         struct nfs_inode *nfsi = NFS_I(inode);
@@ -1327,6 +1282,11 @@ int nfs_commit_inode(struct inode *inode, int how)
         }
         return res;
 }
+#else
+static inline int nfs_commit_list(struct inode *inode, struct list_head *head, int how)
+{
+        return 0;
+}
 #endif
 
 long nfs_sync_mapping_wait(struct address_space *mapping, struct writeback_control *wbc, int how)
@@ -1360,19 +1320,6 @@ long nfs_sync_mapping_wait(struct address_space *mapping, struct writeback_contr
                 ret = nfs_wait_on_requests_locked(inode, idx_start, npages);
                 if (ret != 0)
                         continue;
-                pages = nfs_scan_dirty(mapping, wbc, &head);
-                if (pages != 0) {
-                        spin_unlock(&nfsi->req_lock);
-                        if (how & FLUSH_INVALIDATE) {
-                                nfs_cancel_dirty_list(&head);
-                                ret = pages;
-                        } else
-                                ret = nfs_flush_list(inode, &head, pages, how);
-                        spin_lock(&nfsi->req_lock);
-                        continue;
-                }
-                if (wbc->pages_skipped != 0)
-                        continue;
                 if (nocommit)
                         break;
                 pages = nfs_scan_commit(inode, &head, idx_start, npages);
@@ -1412,7 +1359,7 @@ int nfs_wb_all(struct inode *inode)
         };
         int ret;
 
-        ret = generic_writepages(mapping, &wbc);
+        ret = nfs_writepages(mapping, &wbc);
         if (ret < 0)
                 goto out;
         ret = nfs_sync_mapping_wait(mapping, &wbc, 0);
@@ -1435,11 +1382,9 @@ int nfs_sync_mapping_range(struct address_space *mapping, loff_t range_start, lo
         };
         int ret;
 
-        if (!(how & FLUSH_NOWRITEPAGE)) {
-                ret = generic_writepages(mapping, &wbc);
-                if (ret < 0)
-                        goto out;
-        }
+        ret = nfs_writepages(mapping, &wbc);
+        if (ret < 0)
+                goto out;
         ret = nfs_sync_mapping_wait(mapping, &wbc, how);
         if (ret >= 0)
                 return 0;
@@ -1462,7 +1407,7 @@ int nfs_wb_page_priority(struct inode *inode, struct page *page, int how)
         int ret;
 
         BUG_ON(!PageLocked(page));
-        if (!(how & FLUSH_NOWRITEPAGE) && clear_page_dirty_for_io(page)) {
+        if (clear_page_dirty_for_io(page)) {
                 ret = nfs_writepage_locked(page, &wbc);
                 if (ret < 0)
                         goto out;