On Mon 29-12-14 10:45:22, Linus Torvalds wrote:
> So I think this patch is definitely going in the right direction, but
> at least the __GFP_WRITE handling is insane:
>
> (Patch edited to show the resulting code, without the old deleted lines)
>
> On Mon, Dec 29, 2014 at 9:40 AM, Michal Hocko <mhocko@xxxxxxx> wrote:
> > @@ -1105,13 +1102,11 @@ no_page:
> > if (!page && (fgp_flags & FGP_CREAT)) {
> > int err;
> > if ((fgp_flags & FGP_WRITE) && mapping_cap_account_dirty(mapping))
> > + gfp_mask |= __GFP_WRITE;
> > + if (fgp_flags & FGP_NOFS)
> > + gfp_mask &= ~__GFP_FS;
> >
> > + page = __page_cache_alloc(gfp_mask);
> > if (!page)
> > return NULL;
> >
> > @@ -1122,7 +1117,7 @@ no_page:
> > if (fgp_flags & FGP_ACCESSED)
> > __SetPageReferenced(page);
> >
> > + err = add_to_page_cache_lru(page, mapping, offset, gfp_mask);
>
> Passing __GFP_WRITE into the radix tree allocation routines is not
> sane. So you'd have to mask the bit out again here (unconditionally is
> fine).
Good point!
---
>From 3242f56ae8886a3c605d93960e77176dfe1dff43 Mon Sep 17 00:00:00 2001
From: Michal Hocko <mhocko@xxxxxxx>
Date: Mon, 29 Dec 2014 20:30:35 +0100
Subject: [PATCH] mm: get rid of radix tree gfp mask for pagecache_get_page
2457aec63745 (mm: non-atomically mark page accessed during page cache
allocation where possible) has added a separate parameter for specifying
gfp mask for radix tree allocations.
Not only this is less than optimal from the API point of view
because it is error prone, it is also buggy currently because
grab_cache_page_write_begin is using GFP_KERNEL for radix tree and
if fgp_flags doesn't contain FGP_NOFS (mostly controlled by fs by
AOP_FLAG_NOFS flag) but the mapping_gfp_mask has __GFP_FS cleared then
the radix tree allocation wouldn't obey the restriction and might
recurse into filesystem and cause deadlocks. This is the case for
most filesystems unfortunately because only ext4 and gfs2 are using
AOP_FLAG_NOFS.
Let's simply remove radix_gfp_mask parameter because the allocation
context is same for both page cache and for the radix tree. Just make
sure that the radix tree gets only the sane subset of the mask (e.g. do
not pass __GFP_WRITE).
Long term it is more preferable to convert remaining users of
AOP_FLAG_NOFS to use mapping_gfp_mask instead and simplify this
interface even further.
Reported-by: Dave Chinner <david@xxxxxxxxxxxxx>
Signed-off-by: Michal Hocko <mhocko@xxxxxxx>
---
include/linux/pagemap.h | 13 ++++++-------
mm/filemap.c | 29 ++++++++++++-----------------
2 files changed, 18 insertions(+), 24 deletions(-)
diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index 7ea069cd3257..4b3736f7065c 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -251,7 +251,7 @@ pgoff_t page_cache_prev_hole(struct address_space *mapping,
#define FGP_NOWAIT 0x00000020
struct page *pagecache_get_page(struct address_space *mapping, pgoff_t offset,
- int fgp_flags, gfp_t cache_gfp_mask, gfp_t radix_gfp_mask);
+ int fgp_flags, gfp_t cache_gfp_mask);
/**
* find_get_page - find and get a page reference
@@ -266,13 +266,13 @@ struct page *pagecache_get_page(struct address_space *mapping, pgoff_t offset,
static inline struct page *find_get_page(struct address_space *mapping,
pgoff_t offset)
{
- return pagecache_get_page(mapping, offset, 0, 0, 0);
+ return pagecache_get_page(mapping, offset, 0, 0);
}
static inline struct page *find_get_page_flags(struct address_space *mapping,
pgoff_t offset, int fgp_flags)
{
- return pagecache_get_page(mapping, offset, fgp_flags, 0, 0);
+ return pagecache_get_page(mapping, offset, fgp_flags, 0);
}
/**
@@ -292,7 +292,7 @@ static inline struct page *find_get_page_flags(struct address_space *mapping,
static inline struct page *find_lock_page(struct address_space *mapping,
pgoff_t offset)
{
- return pagecache_get_page(mapping, offset, FGP_LOCK, 0, 0);
+ return pagecache_get_page(mapping, offset, FGP_LOCK, 0);
}
/**
@@ -319,7 +319,7 @@ static inline struct page *find_or_create_page(struct address_space *mapping,
{
return pagecache_get_page(mapping, offset,
FGP_LOCK|FGP_ACCESSED|FGP_CREAT,
- gfp_mask, gfp_mask & GFP_RECLAIM_MASK);
+ gfp_mask);
}
/**
@@ -340,8 +340,7 @@ static inline struct page *grab_cache_page_nowait(struct address_space *mapping,
{
return pagecache_get_page(mapping, index,
FGP_LOCK|FGP_CREAT|FGP_NOFS|FGP_NOWAIT,
- mapping_gfp_mask(mapping),
- GFP_NOFS);
+ mapping_gfp_mask(mapping));
}
struct page *find_get_entry(struct address_space *mapping, pgoff_t offset);
diff --git a/mm/filemap.c b/mm/filemap.c
index e8905bc3cbd7..11477d3b7838 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -1046,8 +1046,7 @@ EXPORT_SYMBOL(find_lock_entry);
* @mapping: the address_space to search
* @offset: the page index
* @fgp_flags: PCG flags
- * @cache_gfp_mask: gfp mask to use for the page cache data page allocation
- * @radix_gfp_mask: gfp mask to use for radix tree node allocation
+ * @gfp_mask: gfp mask to use for the page cache data page allocation
*
* Looks up the page cache slot at @mapping & @offset.
*
@@ -1056,11 +1055,9 @@ EXPORT_SYMBOL(find_lock_entry);
* FGP_ACCESSED: the page will be marked accessed
* FGP_LOCK: Page is return locked
* FGP_CREAT: If page is not present then a new page is allocated using
- * @cache_gfp_mask and added to the page cache and the VM's LRU
- * list. If radix tree nodes are allocated during page cache
- * insertion then @radix_gfp_mask is used. The page is returned
- * locked and with an increased refcount. Otherwise, %NULL is
- * returned.
+ * @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.
*
* If FGP_LOCK or FGP_CREAT are specified then the function may sleep even
* if the GFP flags specified for FGP_CREAT are atomic.
@@ -1068,7 +1065,7 @@ EXPORT_SYMBOL(find_lock_entry);
* If there is a page cache page, it is returned with an increased refcount.
*/
struct page *pagecache_get_page(struct address_space *mapping, pgoff_t offset,
- int fgp_flags, gfp_t cache_gfp_mask, gfp_t radix_gfp_mask)
+ int fgp_flags, gfp_t gfp_mask)
{
struct page *page;
@@ -1105,13 +1102,11 @@ no_page:
if (!page && (fgp_flags & FGP_CREAT)) {
int err;
if ((fgp_flags & FGP_WRITE) && mapping_cap_account_dirty(mapping))
- cache_gfp_mask |= __GFP_WRITE;
- if (fgp_flags & FGP_NOFS) {
- cache_gfp_mask &= ~__GFP_FS;
- radix_gfp_mask &= ~__GFP_FS;
- }
+ gfp_mask |= __GFP_WRITE;
+ if (fgp_flags & FGP_NOFS)
+ gfp_mask &= ~__GFP_FS;
- page = __page_cache_alloc(cache_gfp_mask);
+ page = __page_cache_alloc(gfp_mask);
if (!page)
return NULL;
@@ -1122,7 +1117,8 @@ no_page:
if (fgp_flags & FGP_ACCESSED)
__SetPageReferenced(page);
- err = add_to_page_cache_lru(page, mapping, offset, radix_gfp_mask);
+ err = add_to_page_cache_lru(page, mapping, offset,
+ gfp_mask & GFP_RECLAIM_MASK);
if (unlikely(err)) {
page_cache_release(page);
page = NULL;
@@ -2443,8 +2439,7 @@ struct page *grab_cache_page_write_begin(struct address_space *mapping,
fgp_flags |= FGP_NOFS;
page = pagecache_get_page(mapping, index, fgp_flags,
- mapping_gfp_mask(mapping),
- GFP_KERNEL);
+ mapping_gfp_mask(mapping));
if (page)
wait_for_stable_page(page);
--
2.1.4
--
Michal Hocko
SUSE Labs
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