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Subject[PATCH 2/4] filemap: kill page_cache_read usage in filemap_fault
Date
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.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
---
include/linux/pagemap.h | 1 +
mm/filemap.c | 73 ++++++++++---------------------------------------
2 files changed, 16 insertions(+), 58 deletions(-)

diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index 226f96f0dee0..b13c2442281f 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -252,6 +252,7 @@ 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

struct page *pagecache_get_page(struct address_space *mapping, pgoff_t offset,
int fgp_flags, gfp_t cache_gfp_mask);
diff --git a/mm/filemap.c b/mm/filemap.c
index 81adec8ee02c..f068712c2525 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -1503,6 +1503,9 @@ 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. Otherwise, NULL is returned.
+ * - FGP_FOR_MMAP: Similar to FGP_CREAT, only it unlocks the page after it has
+ * added it to pagecache, as the mmap code expects to do it's own special
+ * locking dance.
*
* If FGP_LOCK or FGP_CREAT are specified then the function may sleep even
* if the GFP flags specified for FGP_CREAT are atomic.
@@ -1555,7 +1558,7 @@ struct page *pagecache_get_page(struct address_space *mapping, pgoff_t offset,
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 */
@@ -1569,6 +1572,13 @@ struct page *pagecache_get_page(struct address_space *mapping, pgoff_t offset,
if (err == -EEXIST)
goto repeat;
}
+
+ /*
+ * add_to_page_cache_lru lock's the page, and for mmap we expect
+ * a unlocked page.
+ */
+ if (fgp_flags & FGP_FOR_MMAP)
+ unlock_page(page);
}

return page;
@@ -2293,39 +2303,6 @@ generic_file_read_iter(struct kiocb *iocb, struct iov_iter *iter)
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.
- */
-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)

/*
@@ -2449,9 +2426,11 @@ 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)) {
@@ -2488,28 +2467,6 @@ vm_fault_t filemap_fault(struct vm_fault *vmf)
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.
--
2.14.3
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