The patch titled
Subject: mm/filemap.c: fix a data race in filemap_fault()
has been added to the -mm tree. Its filename is
mm-filemap-fix-a-data-race-in-filemap_fault.patch
This patch should soon appear at
http://ozlabs.org/~akpm/mmots/broken-out/mm-filemap-fix-a-data-race-in-filemap_fault.patch
and later at
http://ozlabs.org/~akpm/mmotm/broken-out/mm-filemap-fix-a-data-race-in-filemap_fault.patch
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------------------------------------------------------
From: Kirill A. Shutemov <kirill@xxxxxxxxxxxxx>
Subject: mm/filemap.c: fix a data race in filemap_fault()
struct file_ra_state ra.mmap_miss could be accessed concurrently during
page faults as noticed by KCSAN,
BUG: KCSAN: data-race in filemap_fault / filemap_map_pages
write to 0xffff9b1700a2c1b4 of 4 bytes by task 3292 on cpu 30:
filemap_fault+0x920/0xfc0
do_sync_mmap_readahead at mm/filemap.c:2384
(inlined by) filemap_fault at mm/filemap.c:2486
__xfs_filemap_fault+0x112/0x3e0 [xfs]
xfs_filemap_fault+0x74/0x90 [xfs]
__do_fault+0x9e/0x220
do_fault+0x4a0/0x920
__handle_mm_fault+0xc69/0xd00
handle_mm_fault+0xfc/0x2f0
do_page_fault+0x263/0x6f9
page_fault+0x34/0x40
read to 0xffff9b1700a2c1b4 of 4 bytes by task 3313 on cpu 32:
filemap_map_pages+0xc2e/0xd80
filemap_map_pages at mm/filemap.c:2625
do_fault+0x3da/0x920
__handle_mm_fault+0xc69/0xd00
handle_mm_fault+0xfc/0x2f0
do_page_fault+0x263/0x6f9
page_fault+0x34/0x40
Reported by Kernel Concurrency Sanitizer on:
CPU: 32 PID: 3313 Comm: systemd-udevd Tainted: G W L 5.5.0-next-20200210+ #1
Hardware name: HPE ProLiant DL385 Gen10/ProLiant DL385 Gen10, BIOS A40 07/10/2019
ra.mmap_miss is used to contribute the readahead decisions, a data race
could be undesirable. Both the read and write is only under non-exclusive
mmap_sem, two concurrent writers could even underflow the counter. Fix
the underflow by writing to a local variable before committing a final
store to ra.mmap_miss given a small inaccuracy of the counter should be
acceptable.
Link: http://lkml.kernel.org/r/20200211030134.1847-1-cai@xxxxxx
Signed-off-by: Kirill A. Shutemov <kirill@xxxxxxxxxxxxx>
Signed-off-by: Qian Cai <cai@xxxxxx>
Tested-by: Qian Cai <cai@xxxxxx>
Reviewed-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
Cc: Marco Elver <elver@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/filemap.c | 20 +++++++++++++-------
1 file changed, 13 insertions(+), 7 deletions(-)
--- a/mm/filemap.c~mm-filemap-fix-a-data-race-in-filemap_fault
+++ a/mm/filemap.c
@@ -2365,6 +2365,7 @@ static struct file *do_sync_mmap_readahe
struct address_space *mapping = file->f_mapping;
struct file *fpin = NULL;
pgoff_t offset = vmf->pgoff;
+ unsigned int mmap_miss;
/* If we don't want any read-ahead, don't bother */
if (vmf->vma->vm_flags & VM_RAND_READ)
@@ -2380,14 +2381,15 @@ static struct file *do_sync_mmap_readahe
}
/* Avoid banging the cache line if not needed */
- if (ra->mmap_miss < MMAP_LOTSAMISS * 10)
- ra->mmap_miss++;
+ mmap_miss = READ_ONCE(ra->mmap_miss);
+ if (mmap_miss < MMAP_LOTSAMISS * 10)
+ 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.
*/
- if (ra->mmap_miss > MMAP_LOTSAMISS)
+ if (mmap_miss > MMAP_LOTSAMISS)
return fpin;
/*
@@ -2413,13 +2415,15 @@ static struct file *do_async_mmap_readah
struct file_ra_state *ra = &file->f_ra;
struct address_space *mapping = file->f_mapping;
struct file *fpin = NULL;
+ unsigned int mmap_miss;
pgoff_t offset = vmf->pgoff;
/* If we don't want any read-ahead, don't bother */
if (vmf->vma->vm_flags & VM_RAND_READ)
return fpin;
- if (ra->mmap_miss > 0)
- ra->mmap_miss--;
+ mmap_miss = READ_ONCE(ra->mmap_miss);
+ if (mmap_miss)
+ WRITE_ONCE(ra->mmap_miss, --mmap_miss);
if (PageReadahead(page)) {
fpin = maybe_unlock_mmap_for_io(vmf, fpin);
page_cache_async_readahead(mapping, ra, file,
@@ -2586,6 +2590,7 @@ void filemap_map_pages(struct vm_fault *
unsigned long max_idx;
XA_STATE(xas, &mapping->i_pages, start_pgoff);
struct page *page;
+ unsigned int mmap_miss = READ_ONCE(file->f_ra.mmap_miss);
rcu_read_lock();
xas_for_each(&xas, page, end_pgoff) {
@@ -2622,8 +2627,8 @@ void filemap_map_pages(struct vm_fault *
if (page->index >= max_idx)
goto unlock;
- if (file->f_ra.mmap_miss > 0)
- file->f_ra.mmap_miss--;
+ if (mmap_miss > 0)
+ mmap_miss--;
vmf->address += (xas.xa_index - last_pgoff) << PAGE_SHIFT;
if (vmf->pte)
@@ -2643,6 +2648,7 @@ next:
break;
}
rcu_read_unlock();
+ WRITE_ONCE(file->f_ra.mmap_miss, mmap_miss);
}
EXPORT_SYMBOL(filemap_map_pages);
_
Patches currently in -mm which might be from kirill@xxxxxxxxxxxxx are
mm-filemap-fix-a-data-race-in-filemap_fault.patch
