Hi, Dan,
On 10/16/19 3:44 AM, Dan Williams wrote:
On Tue, Oct 15, 2019 at 3:06 AM Kirill A. Shutemov <kirill@xxxxxxxxxxxxx> wrote:
On Tue, Oct 08, 2019 at 11:37:11AM +0200, Thomas Hellström (VMware) wrote:
From: Thomas Hellstrom <thellstrom@xxxxxxxxxx>
A huge pud page can theoretically be faulted in racing with pmd_alloc()
in __handle_mm_fault(). That will lead to pmd_alloc() returning an
invalid pmd pointer. Fix this by adding a pud_trans_unstable() function
similar to pmd_trans_unstable() and check whether the pud is really stable
before using the pmd pointer.
Race:
Thread 1: Thread 2: Comment
create_huge_pud() Fallback - not taken.
create_huge_pud() Taken.
pmd_alloc() Returns an invalid pointer.
Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Fixes: a00cc7d9dd93 ("mm, x86: add support for PUD-sized transparent hugepages")
Signed-off-by: Thomas Hellstrom <thellstrom@xxxxxxxxxx>
---
RFC: We include pud_devmap() as an unstable PUD flag. Is this correct?
Do the same for pmds?
I *think* it is correct and we should do the same for PMD, but I may be
wrong.
Dan, Matthew, could you comment on this?
The _devmap() check in these paths near _trans_unstable() has always
been about avoiding assumptions that the corresponding page might be
page cache or anonymous which for dax it's neither and does not behave
like a typical page.
The concern here is that _trans_huge() returns false for _devmap()
pages, which means that also _trans_unstable() returns false.
Still, I figure someone could zap the entry at any time using madvise(),
so AFAICT the entry is indeed unstable, and it's a bug not to include
_devmap() in the _trans_unstable() functions?
Thanks,
Thomas
