filemap: kill page_cache_read usage in filemap_fault

Patch series "drop the mmap_sem when doing IO in the fault path", v6.

Now that we have proper isolation in place with cgroups2 we have started
going through and fixing the various priority inversions.  Most are all
gone now, but this one is sort of weird since it's not necessarily a
priority inversion that happens within the kernel, but rather because of
something userspace does.

We have giant applications that we want to protect, and parts of these
giant applications do things like watch the system state to determine how
healthy the box is for load balancing and such.  This involves running
'ps' or other such utilities.  These utilities will often walk
/proc/<pid>/whatever, and these files can sometimes need to
down_read(&task->mmap_sem).  Not usually a big deal, but we noticed when
we are stress testing that sometimes our protected application has latency
spikes trying to get the mmap_sem for tasks that are in lower priority
cgroups.

This is because any down_write() on a semaphore essentially turns it into
a mutex, so even if we currently have it held for reading, any new readers
will not be allowed on to keep from starving the writer.  This is fine,
except a lower priority task could be stuck doing IO because it has been
throttled to the point that its IO is taking much longer than normal.  But
because a higher priority group depends on this completing it is now stuck
behind lower priority work.

In order to avoid this particular priority inversion we want to use the
existing retry mechanism to stop from holding the mmap_sem at all if we
are going to do IO.  This already exists in the read case sort of, but
needed to be extended for more than just grabbing the page lock.  With
io.latency we throttle at submit_bio() time, so the readahead stuff can
block and even page_cache_read can block, so all these paths need to have
the mmap_sem dropped.

The other big thing is ->page_mkwrite.  btrfs is particularly shitty here
because we have to reserve space for the dirty page, which can be a very
expensive operation.  We use the same retry method as the read path, and
simply cache the page and verify the page is still setup properly the next
pass through ->page_mkwrite().

I've tested these patches with xfstests and there are no regressions.

This patch (of 3):

If we do not have a page at filemap_fault time we'll do this weird forced
page_cache_read thing to populate the page, and then drop it again and
loop around and find it.  This makes for 2 ways we can read a page in
filemap_fault, and it's not really needed.  Instead add a FGP_FOR_MMAP
flag so that pagecache_get_page() will return a unlocked page that's in
pagecache.  Then use the normal page locking and readpage logic already in
filemap_fault.  This simplifies the no page in page cache case
significantly.

[akpm@linux-foundation.org: fix comment text]
[josef@toxicpanda.com: don't unlock null page in FGP_FOR_MMAP case]
  Link: http://lkml.kernel.org/r/20190312201742.22935-1-josef@toxicpanda.com
Link: http://lkml.kernel.org/r/20181211173801.29535-2-josef@toxicpanda.com
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Tejun Heo <tj@kernel.org>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: "Kirill A. Shutemov" <kirill@shutemov.name>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 15 insertions, 60 deletions

diff --git a/mm/filemap.c b/mm/filemap.c
index ec6566ffbd90..64d014f940e9 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -1587,6 +1587,9 @@ EXPORT_SYMBOL(find_lock_entry);
  *   @gfp_mask and added to the page cache and the VM's LRU
  *   list. The page is returned locked and with an increased
  *   refcount.
+ * - FGP_FOR_MMAP: Similar to FGP_CREAT, only we want to allow the caller to do
+ *   its own locking dance if the page is already in cache, or unlock the page
+ *   before returning if we had to add the page to pagecache.
  *
  * If FGP_LOCK or FGP_CREAT are specified then the function may sleep even
  * if the GFP flags specified for FGP_CREAT are atomic.
@@ -1641,7 +1644,7 @@ no_page:
                 if (!page)
                         return NULL;
 
-                if (WARN_ON_ONCE(!(fgp_flags & FGP_LOCK)))
+                if (WARN_ON_ONCE(!(fgp_flags & (FGP_LOCK | FGP_FOR_MMAP))))
                         fgp_flags |= FGP_LOCK;
 
                 /* Init accessed so avoid atomic mark_page_accessed later */
@@ -1655,6 +1658,13 @@ no_page:
                         if (err == -EEXIST)
                                 goto repeat;
                 }
+
+                /*
+                 * add_to_page_cache_lru locks the page, and for mmap we expect
+                 * an unlocked page.
+                 */
+                if (page && (fgp_flags & FGP_FOR_MMAP))
+                        unlock_page(page);
         }
 
         return page;
@@ -2379,41 +2389,6 @@ out:
 EXPORT_SYMBOL(generic_file_read_iter);
 
 #ifdef CONFIG_MMU
-/**
- * page_cache_read - adds requested page to the page cache if not already there
- * @file:       file to read
- * @offset:     page index
- * @gfp_mask:   memory allocation flags
- *
- * This adds the requested page to the page cache if it isn't already there,
- * and schedules an I/O to read in its contents from disk.
- *
- * Return: %0 on success, negative error code otherwise.
- */
-static int page_cache_read(struct file *file, pgoff_t offset, gfp_t gfp_mask)
-{
-        struct address_space *mapping = file->f_mapping;
-        struct page *page;
-        int ret;
-
-        do {
-                page = __page_cache_alloc(gfp_mask);
-                if (!page)
-                        return -ENOMEM;
-
-                ret = add_to_page_cache_lru(page, mapping, offset, gfp_mask);
-                if (ret == 0)
-                        ret = mapping->a_ops->readpage(file, page);
-                else if (ret == -EEXIST)
-                        ret = 0; /* losing race to add is OK */
-
-                put_page(page);
-
-        } while (ret == AOP_TRUNCATED_PAGE);
-
-        return ret;
-}
-
 #define MMAP_LOTSAMISS  (100)
 
 /*
@@ -2539,9 +2514,11 @@ vm_fault_t filemap_fault(struct vm_fault *vmf)
                 count_memcg_event_mm(vmf->vma->vm_mm, PGMAJFAULT);
                 ret = VM_FAULT_MAJOR;
 retry_find:
-                page = find_get_page(mapping, offset);
+                page = pagecache_get_page(mapping, offset,
+                                          FGP_CREAT|FGP_FOR_MMAP,
+                                          vmf->gfp_mask);
                 if (!page)
-                        goto no_cached_page;
+                        return vmf_error(-ENOMEM);
         }
 
         if (!lock_page_or_retry(page, vmf->vma->vm_mm, vmf->flags)) {
@@ -2578,28 +2555,6 @@ retry_find:
         vmf->page = page;
         return ret | VM_FAULT_LOCKED;
 
-no_cached_page:
-        /*
-         * We're only likely to ever get here if MADV_RANDOM is in
-         * effect.
-         */
-        error = page_cache_read(file, offset, vmf->gfp_mask);
-
-        /*
-         * The page we want has now been added to the page cache.
-         * In the unlikely event that someone removed it in the
-         * meantime, we'll just come back here and read it again.
-         */
-        if (error >= 0)
-                goto retry_find;
-
-        /*
-         * An error return from page_cache_read can result if the
-         * system is low on memory, or a problem occurs while trying
-         * to schedule I/O.
-         */
-        return vmf_error(error);
-
 page_not_uptodate:
         /*
          * Umm, take care of errors if the page isn't up-to-date.