readahead: clean up and simplify the code for filemap page fault readahead

This shouldn't really change behavior all that much, but the single rather
complex function with read-ahead inside a loop etc is broken up into more
manageable pieces.

The behaviour is also less subtle, with the read-ahead being done up-front
rather than inside some subtle loop and thus avoiding the now unnecessary
extra state variables (ie "did_readaround" is gone).

Fengguang: the code split in fact fixed a bug reported by Pavel Levshin:
the PGMAJFAULT accounting used to be bypassed when MADV_RANDOM is set, in
which case the original code will directly jump to no_cached_page reading.

Cc: Pavel Levshin <lpk@581.spb.su>
Cc: <wli@movementarian.org>
Cc: Nick Piggin <npiggin@suse.de>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 89 insertions, 67 deletions

diff --git a/mm/filemap.c b/mm/filemap.c
index dcef9fd6b92e..827536485599 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -1456,6 +1456,68 @@ static int page_cache_read(struct file *file, pgoff_t offset)
 
 #define MMAP_LOTSAMISS  (100)
 
+/*
+ * Synchronous readahead happens when we don't even find
+ * a page in the page cache at all.
+ */
+static void do_sync_mmap_readahead(struct vm_area_struct *vma,
+                                   struct file_ra_state *ra,
+                                   struct file *file,
+                                   pgoff_t offset)
+{
+        unsigned long ra_pages;
+        struct address_space *mapping = file->f_mapping;
+
+        /* If we don't want any read-ahead, don't bother */
+        if (VM_RandomReadHint(vma))
+                return;
+
+        if (VM_SequentialReadHint(vma)) {
+                page_cache_sync_readahead(mapping, ra, file, offset, 1);
+                return;
+        }
+
+        if (ra->mmap_miss < INT_MAX)
+                ra->mmap_miss++;
+
+        /*
+         * Do we miss much more than hit in this file? If so,
+         * stop bothering with read-ahead. It will only hurt.
+         */
+        if (ra->mmap_miss > MMAP_LOTSAMISS)
+                return;
+
+        ra_pages = max_sane_readahead(ra->ra_pages);
+        if (ra_pages) {
+                pgoff_t start = 0;
+
+                if (offset > ra_pages / 2)
+                        start = offset - ra_pages / 2;
+                do_page_cache_readahead(mapping, file, start, ra_pages);
+        }
+}
+
+/*
+ * Asynchronous readahead happens when we find the page and PG_readahead,
+ * so we want to possibly extend the readahead further..
+ */
+static void do_async_mmap_readahead(struct vm_area_struct *vma,
+                                    struct file_ra_state *ra,
+                                    struct file *file,
+                                    struct page *page,
+                                    pgoff_t offset)
+{
+        struct address_space *mapping = file->f_mapping;
+
+        /* If we don't want any read-ahead, don't bother */
+        if (VM_RandomReadHint(vma))
+                return;
+        if (ra->mmap_miss > 0)
+                ra->mmap_miss--;
+        if (PageReadahead(page))
+                page_cache_async_readahead(mapping, ra, file, page, offset, 1);
+}
+
 /**
  * filemap_fault - read in file data for page fault handling
  * @vma:        vma in which the fault was taken
@@ -1475,78 +1537,44 @@ int filemap_fault(struct vm_area_struct *vma, struct vm_fault *vmf)
         struct address_space *mapping = file->f_mapping;
         struct file_ra_state *ra = &file->f_ra;
         struct inode *inode = mapping->host;
+        pgoff_t offset = vmf->pgoff;
         struct page *page;
         pgoff_t size;
-        int did_readaround = 0;
         int ret = 0;
 
         size = (i_size_read(inode) + PAGE_CACHE_SIZE - 1) >> PAGE_CACHE_SHIFT;
-        if (vmf->pgoff >= size)
+        if (offset >= size)
                 return VM_FAULT_SIGBUS;
 
-        /* If we don't want any read-ahead, don't bother */
-        if (VM_RandomReadHint(vma))
-                goto no_cached_page;
-
         /*
          * Do we have something in the page cache already?
          */
-retry_find:
-        page = find_lock_page(mapping, vmf->pgoff);
-        /*
-         * For sequential accesses, we use the generic readahead logic.
-         */
-        if (VM_SequentialReadHint(vma)) {
-                if (!page) {
-                        page_cache_sync_readahead(mapping, ra, file,
-                                                           vmf->pgoff, 1);
-                        page = find_lock_page(mapping, vmf->pgoff);
-                        if (!page)
-                                goto no_cached_page;
-                }
-                if (PageReadahead(page)) {
-                        page_cache_async_readahead(mapping, ra, file, page,
-                                                           vmf->pgoff, 1);
-                }
-        }
-
-        if (!page) {
-                unsigned long ra_pages;
-
-                ra->mmap_miss++;
-
+        page = find_get_page(mapping, offset);
+        if (likely(page)) {
                 /*
-                 * Do we miss much more than hit in this file? If so,
-                 * stop bothering with read-ahead. It will only hurt.
+                 * We found the page, so try async readahead before
+                 * waiting for the lock.
                  */
-                if (ra->mmap_miss > MMAP_LOTSAMISS)
-                        goto no_cached_page;
+                do_async_mmap_readahead(vma, ra, file, page, offset);
+                lock_page(page);
 
-                /*
-                 * To keep the pgmajfault counter straight, we need to
-                 * check did_readaround, as this is an inner loop.
-                 */
-                if (!did_readaround) {
-                        ret = VM_FAULT_MAJOR;
-                        count_vm_event(PGMAJFAULT);
-                }
-                did_readaround = 1;
-                ra_pages = max_sane_readahead(file->f_ra.ra_pages);
-                if (ra_pages) {
-                        pgoff_t start = 0;
-
-                        if (vmf->pgoff > ra_pages / 2)
-                                start = vmf->pgoff - ra_pages / 2;
-                        do_page_cache_readahead(mapping, file, start, ra_pages);
+                /* Did it get truncated? */
+                if (unlikely(page->mapping != mapping)) {
+                        unlock_page(page);
+                        put_page(page);
+                        goto no_cached_page;
                 }
-                page = find_lock_page(mapping, vmf->pgoff);
+        } else {
+                /* No page in the page cache at all */
+                do_sync_mmap_readahead(vma, ra, file, offset);
+                count_vm_event(PGMAJFAULT);
+                ret = VM_FAULT_MAJOR;
+retry_find:
+                page = find_lock_page(mapping, offset);
                 if (!page)
                         goto no_cached_page;
         }
 
-        if (!did_readaround)
-                ra->mmap_miss--;
-
         /*
          * We have a locked page in the page cache, now we need to check
          * that it's up-to-date. If not, it is going to be due to an error.
@@ -1554,18 +1582,18 @@ retry_find:
         if (unlikely(!PageUptodate(page)))
                 goto page_not_uptodate;
 
-        /* Must recheck i_size under page lock */
+        /*
+         * Found the page and have a reference on it.
+         * We must recheck i_size under page lock.
+         */
         size = (i_size_read(inode) + PAGE_CACHE_SIZE - 1) >> PAGE_CACHE_SHIFT;
-        if (unlikely(vmf->pgoff >= size)) {
+        if (unlikely(offset >= size)) {
                 unlock_page(page);
                 page_cache_release(page);
                 return VM_FAULT_SIGBUS;
         }
 
-        /*
-         * Found the page and have a reference on it.
-         */
-        ra->prev_pos = (loff_t)page->index << PAGE_CACHE_SHIFT;
+        ra->prev_pos = (loff_t)offset << PAGE_CACHE_SHIFT;
         vmf->page = page;
         return ret | VM_FAULT_LOCKED;
 
@@ -1574,7 +1602,7 @@ no_cached_page:
          * We're only likely to ever get here if MADV_RANDOM is in
          * effect.
          */
-        error = page_cache_read(file, vmf->pgoff);
+        error = page_cache_read(file, offset);
 
         /*
          * The page we want has now been added to the page cache.
@@ -1594,12 +1622,6 @@ no_cached_page:
         return VM_FAULT_SIGBUS;
 
 page_not_uptodate:
-        /* IO error path */
-        if (!did_readaround) {
-                ret = VM_FAULT_MAJOR;
-                count_vm_event(PGMAJFAULT);
-        }
-
         /*
          * Umm, take care of errors if the page isn't up-to-date.
          * Try to re-read it _once_. We do this synchronously,