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futex: Fix uninterruptible loop due to gate_area

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It was found (by Sasha) that if you use a futex located in the gate
area we get stuck in an uninterruptible infinite loop, much like the
ZERO_PAGE issue.

While looking at this problem, PeterZ realized you'll get into similar
trouble when hitting any install_special_pages() mapping.  And are there
still drivers setting up their own special mmaps without page->mapping,
and without special VM or pte flags to make get_user_pages fail?

In most cases, if page->mapping is NULL, we do not need to retry at all:
Linus points out that even /proc/sys/vm/drop_caches poses no problem,
because it ends up using remove_mapping(), which takes care not to
interfere when the page reference count is raised.

But there is still one case which does need a retry: if memory pressure
called shmem_writepage in between get_user_pages_fast dropping page
table lock and our acquiring page lock, then the page gets switched from
filecache to swapcache (and ->mapping set to NULL) whatever the refcount.
Fault it back in to get the page->mapping needed for key->shared.inode.

Reported-by: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Hugh Dickins <hughd@google.com>
Cc: stable@vger.kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Hugh Dickins 2011-12-31 11:44:01 -08:00 committed by Linus Torvalds
parent 06867fbb8a
commit e6780f7243
1 changed files with 20 additions and 8 deletions
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26
kernel/futex.c
View file

@ -314,17 +314,29 @@ get_futex_key(u32 __user *uaddr, int fshared, union futex_key *key, int rw)
#endif
lock_page(page_head);
/*
* If page_head->mapping is NULL, then it cannot be a PageAnon
* page; but it might be the ZERO_PAGE or in the gate area or
* in a special mapping (all cases which we are happy to fail);
* or it may have been a good file page when get_user_pages_fast
* found it, but truncated or holepunched or subjected to
* invalidate_complete_page2 before we got the page lock (also
* cases which we are happy to fail). And we hold a reference,
* so refcount care in invalidate_complete_page's remove_mapping
* prevents drop_caches from setting mapping to NULL beneath us.
*
* The case we do have to guard against is when memory pressure made
* shmem_writepage move it from filecache to swapcache beneath us:
* an unlikely race, but we do need to retry for page_head->mapping.
*/
if (!page_head->mapping) {
int shmem_swizzled = PageSwapCache(page_head);
unlock_page(page_head);
put_page(page_head);
/*
* ZERO_PAGE pages don't have a mapping. Avoid a busy loop
* trying to find one. RW mapping would have COW'd (and thus
* have a mapping) so this page is RO and won't ever change.
*/
if ((page_head == ZERO_PAGE(address)))
return -EFAULT;
if (shmem_swizzled)
goto again;
return -EFAULT;
}
/*