| Message ID | alpine.LSU.2.11.2008161338360.20413@eggly.anvils (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | uprobes: __replace_page() avoid BUG in munlock_vma_page() | expand |
> On Aug 16, 2020, at 1:44 PM, Hugh Dickins <hughd@google.com> wrote: > > syzbot crashed on the VM_BUG_ON_PAGE(PageTail) in munlock_vma_page(), > when called from uprobes __replace_page(). Which of many ways to fix it? > Settled on not calling when PageCompound (since Head and Tail are equals > in this context, PageCompound the usual check in uprobes.c, and the prior > use of FOLL_SPLIT_PMD will have cleared PageMlocked already). > > Reported-by: syzbot <syzkaller@googlegroups.com> > Fixes: 5a52c9df62b4 ("uprobe: use FOLL_SPLIT_PMD instead of FOLL_SPLIT") > Signed-off-by: Hugh Dickins <hughd@google.com> > Cc: stable@vger.kernel.org # v5.4+ > --- > This one is not a 5.9-rc regression, but still good to fix. > > kernel/events/uprobes.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > --- v5.9-rc/kernel/events/uprobes.c 2020-08-12 19:46:50.851196584 -0700 > +++ linux/kernel/events/uprobes.c 2020-08-16 13:18:35.292821674 -0700 > @@ -205,7 +205,7 @@ static int __replace_page(struct vm_area > try_to_free_swap(old_page); > page_vma_mapped_walk_done(&pvmw); > > - if (vma->vm_flags & VM_LOCKED) > + if ((vma->vm_flags & VM_LOCKED) && !PageCompound(old_page)) Do we need munlock_vma_page() for THP page head? Thanks, Song > munlock_vma_page(old_page); > put_page(old_page); >
On Mon, 17 Aug 2020, Song Liu wrote: > > On Aug 16, 2020, at 1:44 PM, Hugh Dickins <hughd@google.com> wrote: > > > > syzbot crashed on the VM_BUG_ON_PAGE(PageTail) in munlock_vma_page(), > > when called from uprobes __replace_page(). Which of many ways to fix it? > > Settled on not calling when PageCompound (since Head and Tail are equals > > in this context, PageCompound the usual check in uprobes.c, and the prior > > use of FOLL_SPLIT_PMD will have cleared PageMlocked already). > > > > Reported-by: syzbot <syzkaller@googlegroups.com> > > Fixes: 5a52c9df62b4 ("uprobe: use FOLL_SPLIT_PMD instead of FOLL_SPLIT") > > Signed-off-by: Hugh Dickins <hughd@google.com> > > Cc: stable@vger.kernel.org # v5.4+ > > --- > > This one is not a 5.9-rc regression, but still good to fix. > > > > kernel/events/uprobes.c | 2 +- > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > --- v5.9-rc/kernel/events/uprobes.c 2020-08-12 19:46:50.851196584 -0700 > > +++ linux/kernel/events/uprobes.c 2020-08-16 13:18:35.292821674 -0700 > > @@ -205,7 +205,7 @@ static int __replace_page(struct vm_area > > try_to_free_swap(old_page); > > page_vma_mapped_walk_done(&pvmw); > > > > - if (vma->vm_flags & VM_LOCKED) > > + if ((vma->vm_flags & VM_LOCKED) && !PageCompound(old_page)) > > Do we need munlock_vma_page() for THP page head? No: as the commit message says "the prior use of FOLL_SPLIT_PMD will have cleared PageMlocked already" - our THP implementation has difficulty supporting Mlocked consistently when the huge page is somewhere mapped by ptes, so one of the things that __split_huge_pmd() does is clear_page_mlock(), then PageDoubleMap will prevent Mlocked being set again once GUP has brought the old_page pte back in. But if you'd prefer us to munlock_vma_page(compound_head(old_page)) instead, I can certainly change the patch: it's one of the options I considered, but couldn't quite bring myself to do it that way, knowing that actually it would never find PageMlocked set. (If PageMlocked were allowed on tail pages, I'd have used a PageMlocked test instead of the PageCompound one: I spent nearly an hour bikeshedding the alternatives here!) (One day I must remind myself of when munlock_vma_page() should be used, versus when clear_page_mlock() should be used: I think it comes down to a choice of which stats get incremented, but I may also be forgetting something more important: anyway, no obvious reason to depart from the munlock_vma_page() that's always been used here.) Hugh
On 08/16, Hugh Dickins wrote: > > --- v5.9-rc/kernel/events/uprobes.c 2020-08-12 19:46:50.851196584 -0700 > +++ linux/kernel/events/uprobes.c 2020-08-16 13:18:35.292821674 -0700 > @@ -205,7 +205,7 @@ static int __replace_page(struct vm_area > try_to_free_swap(old_page); > page_vma_mapped_walk_done(&pvmw); > > - if (vma->vm_flags & VM_LOCKED) > + if ((vma->vm_flags & VM_LOCKED) && !PageCompound(old_page)) > munlock_vma_page(old_page); > put_page(old_page); Thanks! Acked-by: Oleg Nesterov <oleg@redhat.com>
* Hugh Dickins <hughd@google.com> [2020-08-16 13:44:25]: > syzbot crashed on the VM_BUG_ON_PAGE(PageTail) in munlock_vma_page(), > when called from uprobes __replace_page(). Which of many ways to fix it? > Settled on not calling when PageCompound (since Head and Tail are equals > in this context, PageCompound the usual check in uprobes.c, and the prior > use of FOLL_SPLIT_PMD will have cleared PageMlocked already). > > Reported-by: syzbot <syzkaller@googlegroups.com> > Fixes: 5a52c9df62b4 ("uprobe: use FOLL_SPLIT_PMD instead of FOLL_SPLIT") > Signed-off-by: Hugh Dickins <hughd@google.com> > Cc: stable@vger.kernel.org # v5.4+ > --- > This one is not a 5.9-rc regression, but still good to fix. > > kernel/events/uprobes.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > --- v5.9-rc/kernel/events/uprobes.c 2020-08-12 19:46:50.851196584 -0700 > +++ linux/kernel/events/uprobes.c 2020-08-16 13:18:35.292821674 -0700 > @@ -205,7 +205,7 @@ static int __replace_page(struct vm_area > try_to_free_swap(old_page); > page_vma_mapped_walk_done(&pvmw); > > - if (vma->vm_flags & VM_LOCKED) > + if ((vma->vm_flags & VM_LOCKED) && !PageCompound(old_page)) > munlock_vma_page(old_page); > put_page(old_page); > Looks good to me. Reviewed-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
> On Aug 17, 2020, at 12:17 AM, Hugh Dickins <hughd@google.com> wrote: > > On Mon, 17 Aug 2020, Song Liu wrote: >>> On Aug 16, 2020, at 1:44 PM, Hugh Dickins <hughd@google.com> wrote: >>> >>> syzbot crashed on the VM_BUG_ON_PAGE(PageTail) in munlock_vma_page(), >>> when called from uprobes __replace_page(). Which of many ways to fix it? >>> Settled on not calling when PageCompound (since Head and Tail are equals >>> in this context, PageCompound the usual check in uprobes.c, and the prior >>> use of FOLL_SPLIT_PMD will have cleared PageMlocked already). >>> >>> Reported-by: syzbot <syzkaller@googlegroups.com> >>> Fixes: 5a52c9df62b4 ("uprobe: use FOLL_SPLIT_PMD instead of FOLL_SPLIT") >>> Signed-off-by: Hugh Dickins <hughd@google.com> >>> Cc: stable@vger.kernel.org # v5.4+ >>> --- >>> This one is not a 5.9-rc regression, but still good to fix. >>> >>> kernel/events/uprobes.c | 2 +- >>> 1 file changed, 1 insertion(+), 1 deletion(-) >>> >>> --- v5.9-rc/kernel/events/uprobes.c 2020-08-12 19:46:50.851196584 -0700 >>> +++ linux/kernel/events/uprobes.c 2020-08-16 13:18:35.292821674 -0700 >>> @@ -205,7 +205,7 @@ static int __replace_page(struct vm_area >>> try_to_free_swap(old_page); >>> page_vma_mapped_walk_done(&pvmw); >>> >>> - if (vma->vm_flags & VM_LOCKED) >>> + if ((vma->vm_flags & VM_LOCKED) && !PageCompound(old_page)) >> >> Do we need munlock_vma_page() for THP page head? > > No: as the commit message says "the prior use of FOLL_SPLIT_PMD > will have cleared PageMlocked already" - our THP implementation > has difficulty supporting Mlocked consistently when the huge page is > somewhere mapped by ptes, so one of the things that __split_huge_pmd() > does is clear_page_mlock(), then PageDoubleMap will prevent Mlocked > being set again once GUP has brought the old_page pte back in. > > But if you'd prefer us to munlock_vma_page(compound_head(old_page)) > instead, I can certainly change the patch: it's one of the options > I considered, but couldn't quite bring myself to do it that way, > knowing that actually it would never find PageMlocked set. (If > PageMlocked were allowed on tail pages, I'd have used a PageMlocked > test instead of the PageCompound one: I spent nearly an hour > bikeshedding the alternatives here!) > > (One day I must remind myself of when munlock_vma_page() should be > used, versus when clear_page_mlock() should be used: I think it comes > down to a choice of which stats get incremented, but I may also be > forgetting something more important: anyway, no obvious reason to > depart from the munlock_vma_page() that's always been used here.) Thanks a lot for the explanation! Acked-by: Song Liu <songliubraving@fb.com>
--- v5.9-rc/kernel/events/uprobes.c 2020-08-12 19:46:50.851196584 -0700 +++ linux/kernel/events/uprobes.c 2020-08-16 13:18:35.292821674 -0700 @@ -205,7 +205,7 @@ static int __replace_page(struct vm_area try_to_free_swap(old_page); page_vma_mapped_walk_done(&pvmw); - if (vma->vm_flags & VM_LOCKED) + if ((vma->vm_flags & VM_LOCKED) && !PageCompound(old_page)) munlock_vma_page(old_page); put_page(old_page);
syzbot crashed on the VM_BUG_ON_PAGE(PageTail) in munlock_vma_page(), when called from uprobes __replace_page(). Which of many ways to fix it? Settled on not calling when PageCompound (since Head and Tail are equals in this context, PageCompound the usual check in uprobes.c, and the prior use of FOLL_SPLIT_PMD will have cleared PageMlocked already). Reported-by: syzbot <syzkaller@googlegroups.com> Fixes: 5a52c9df62b4 ("uprobe: use FOLL_SPLIT_PMD instead of FOLL_SPLIT") Signed-off-by: Hugh Dickins <hughd@google.com> Cc: stable@vger.kernel.org # v5.4+ --- This one is not a 5.9-rc regression, but still good to fix. kernel/events/uprobes.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)