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Subject[PATCH] mm/filemap.c: Always read one page in do_sync_mmap_readahead()
Date
filemap_fault() calls do_sync_mmap_readahead() to read pages when no
page is found in the page cache. However do_sync_mmap_readahead() will
not actually read any pages if VM_RAND_READ is specified or if there
have been a lot of page cache misses.

This means filemap_fault() will see a folio that is not up-to-date which
is treated as an IO error. The IO error handling path happens to make
VM_RAND_READ work as expected though because it retries reading the
page.

However it would be cleaner if this was handled in
do_sync_mmap_readahead() to match what is done for VM_HUGEPAGE. Also as
do_sync_mmap_readahead() adds the newly allocated folio to the pagecache
and unlocks it this clean-up also allows the FGP_FOR_MMAP flag to be
removed.

Signed-off-by: Alistair Popple <apopple@nvidia.com>
Suggested-by: Matthew Wilcox <willy@infradead.org>
---
include/linux/pagemap.h | 7 +++---
mm/filemap.c | 47 +++++++++++++----------------------------
2 files changed, 18 insertions(+), 36 deletions(-)

diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index 993994cd943a..e0e0f5e7d4a0 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -505,10 +505,9 @@ pgoff_t page_cache_prev_miss(struct address_space *mapping,
#define FGP_WRITE 0x00000008
#define FGP_NOFS 0x00000010
#define FGP_NOWAIT 0x00000020
-#define FGP_FOR_MMAP 0x00000040
-#define FGP_HEAD 0x00000080
-#define FGP_ENTRY 0x00000100
-#define FGP_STABLE 0x00000200
+#define FGP_HEAD 0x00000040
+#define FGP_ENTRY 0x00000080
+#define FGP_STABLE 0x00000100

struct folio *__filemap_get_folio(struct address_space *mapping, pgoff_t index,
int fgp_flags, gfp_t gfp);
diff --git a/mm/filemap.c b/mm/filemap.c
index 9a1eef6c5d35..15d7e0a0ad4b 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -1922,9 +1922,6 @@ static void *mapping_get_entry(struct address_space *mapping, pgoff_t index)
* * %FGP_CREAT - If no page is present then a new page is allocated using
* @gfp 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 - The caller wants to do its own locking dance if the
- * page is already in cache. If the page was allocated, unlock it before
- * returning so the caller can do the same dance.
* * %FGP_WRITE - The page will be written to by the caller.
* * %FGP_NOFS - __GFP_FS will get cleared in gfp.
* * %FGP_NOWAIT - Don't get blocked by page lock.
@@ -1993,7 +1990,7 @@ struct folio *__filemap_get_folio(struct address_space *mapping, pgoff_t index,
if (!folio)
return NULL;

- if (WARN_ON_ONCE(!(fgp_flags & (FGP_LOCK | FGP_FOR_MMAP))))
+ if (WARN_ON_ONCE(!(fgp_flags & FGP_LOCK)))
fgp_flags |= FGP_LOCK;

/* Init accessed so avoid atomic mark_page_accessed later */
@@ -2007,13 +2004,6 @@ struct folio *__filemap_get_folio(struct address_space *mapping, pgoff_t index,
if (err == -EEXIST)
goto repeat;
}
-
- /*
- * filemap_add_folio locks the page, and for mmap
- * we expect an unlocked page.
- */
- if (folio && (fgp_flags & FGP_FOR_MMAP))
- folio_unlock(folio);
}

return folio;
@@ -3011,14 +3001,8 @@ static struct file *do_sync_mmap_readahead(struct vm_fault *vmf)
}
#endif

- /* If we don't want any read-ahead, don't bother */
- if (vmf->vma->vm_flags & VM_RAND_READ)
- return fpin;
- if (!ra->ra_pages)
- return fpin;
-
+ fpin = maybe_unlock_mmap_for_io(vmf, fpin);
if (vmf->vma->vm_flags & VM_SEQ_READ) {
- fpin = maybe_unlock_mmap_for_io(vmf, fpin);
page_cache_sync_ra(&ractl, ra->ra_pages);
return fpin;
}
@@ -3029,19 +3013,20 @@ static struct file *do_sync_mmap_readahead(struct vm_fault *vmf)
WRITE_ONCE(ra->mmap_miss, ++mmap_miss);

/*
- * Do we miss much more than hit in this file? If so,
- * stop bothering with read-ahead. It will only hurt.
+ * mmap read-around. If we don't want any read-ahead or if we miss more
+ * than we hit don't bother with read-ahead and just read a single page.
*/
- if (mmap_miss > MMAP_LOTSAMISS)
- return fpin;
+ if ((vmf->vma->vm_flags & VM_RAND_READ) ||
+ !ra->ra_pages || mmap_miss > MMAP_LOTSAMISS) {
+ ra->start = vmf->pgoff;
+ ra->size = 1;
+ ra->async_size = 0;
+ } else {
+ ra->start = max_t(long, 0, vmf->pgoff - ra->ra_pages / 2);
+ ra->size = ra->ra_pages;
+ ra->async_size = ra->ra_pages / 4;
+ }

- /*
- * mmap read-around
- */
- fpin = maybe_unlock_mmap_for_io(vmf, fpin);
- ra->start = max_t(long, 0, vmf->pgoff - ra->ra_pages / 2);
- ra->size = ra->ra_pages;
- ra->async_size = ra->ra_pages / 4;
ractl._index = ra->start;
page_cache_ra_order(&ractl, ra, 0);
return fpin;
@@ -3145,9 +3130,7 @@ vm_fault_t filemap_fault(struct vm_fault *vmf)
filemap_invalidate_lock_shared(mapping);
mapping_locked = true;
}
- folio = __filemap_get_folio(mapping, index,
- FGP_CREAT|FGP_FOR_MMAP,
- vmf->gfp_mask);
+ folio = filemap_get_folio(mapping, index);
if (!folio) {
if (fpin)
goto out_retry;
--
2.35.1
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