On 4 Nov 2017, at 23:01, huang ying wrote:
On Fri, Nov 3, 2017 at 11:00 PM, Zi Yan <zi.yan@xxxxxxxxxxxxxx> wrote:
On 3 Nov 2017, at 3:52, Huang, Ying wrote:
From: Huang Ying <ying.huang@xxxxxxxxx>
If THP migration is enabled, the following situation is possible,
- A THP is mapped at source address
- Migration is started to move the THP to another node
- Page fault occurs
- The PMD (migration entry) is copied to the destination address in
mremap
You mean the page fault path follows the source address and sees
pmd_none() now
because mremap() clears it and remaps the page with dest address.
Otherwise, it seems not possible to get into handle_userfault(),
since it is called in
pmd_none() branch inside do_huge_pmd_anonymous_page().
That is, it is possible for handle_userfault() encounter a PMD entry
which has been handled but !pmd_present(). In the current
implementation, we will wait for such PMD entries, which may cause
unnecessary waiting, and potential soft lockup.
handle_userfault() should only see pmd_none() in the situation you
describe,
whereas !pmd_present() (migration entry case) should lead to
pmd_migration_entry_wait().
Yes. This is my understanding of the source code too. And I
described it in the original patch description too. I just want to
make sure whether it is possible that !pmd_none() and !pmd_present()
for a PMD in userfaultfd_must_wait(). And, whether it is possible for
us to implement PMD mapping copying in UFFDIO_COPY in the future?
Thanks for clarifying it. We both agree that !pmd_present(), which means
PMD migration entry, does not get into userfaultfd_must_wait(),
then there seems to be no issue with current code yet.
However, the if (!pmd_present(_pmd)) in userfaultfd_must_wait() does not
match
the exact condition. How about the patch below? It can catch pmd
migration entries,
which are only possible in x86_64 at the moment.
diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index 1c713fd5b3e6..dda25444a6ee 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -294,9 +294,11 @@ static inline bool userfaultfd_must_wait(struct
userfaultfd_ctx *ctx,
* pmd_trans_unstable) of the pmd.
*/
_pmd = READ_ONCE(*pmd);
- if (!pmd_present(_pmd))
+ if (pmd_none(_pmd))
goto out;
+ VM_BUG_ON(thp_migration_supported() &&
is_pmd_migration_entry(_pmd));
+
ret = false;
if (pmd_trans_huge(_pmd))
goto out;
—
Best Regards,
Yan Zi
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