Hi, Nadav,
On Sun, Dec 20, 2020 at 12:06:38AM -0800, Nadav Amit wrote:
[...]
So to correct myself, I think that what I really encountered was actually during MM_CP_UFFD_WP_RESOLVE (i.e., when the protection is removed). The problem was that in this case the “write”-bit was removed during unprotect. Sorry for the strange formatting to fit within 80 columns:
I assume I can ignore the race mentioned in the commit message but only refer to this one below. However I'm still confused. Please see below.
[ Start: PTE is writable ]
cpu0 cpu1 cpu2
[ Writable PTE cached in TLB ]
Here cpu2 got writable pte in tlb. But why?
If below is an unprotect, it means it must have been protected once by userfaultfd, right? If so, the previous change_protection_range() which did the wr-protect should have done a tlb flush already before it returns (since pages>0 - we protected one pte at least). Then I can't see why cpu2 tlb has stall data.
If I assume cpu2 doesn't have that cached tlb, then "write to old page" won't happen either, because cpu1/cpu2 will all go through the cow path and pgtable lock should serialize them.
userfaultfd_writeprotect() [ write-*unprotect* ] mwriteprotect_range() mmap_read_lock() change_protection()
change_protection_range() ... change_pte_range() [ *clear* “write”-bit ] [ defer TLB flushes] [ page-fault ] … wp_page_copy() cow_user_page() [ copy page ] [ write to old page ] … set_pte_at_notify()
[ End: cpu2 write not copied form old to new page. ]
Could you share how to reproduce the problem? I would be glad to give it a shot as well.
PS: Sorry to not have read the other series of yours. It seems to need some chunk of time so I postponed it a bit due to other things; but I'll read at least the fixes very soon.
Thanks,