Messages in this thread Patch in this message | | | Date | Sun, 22 Jun 2008 10:26:24 -0700 (PDT) | From | Linus Torvalds <> | Subject | Re: Linux 2.6.26-rc7 |
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On Sun, 22 Jun 2008, Linus Torvalds wrote: > > Hmm. Can you see which part of it broke? Was it the "fix XIP" part ot the > ZERO_PAGE part? The easiest way to test is to apply this patch, and see > (it just disables the XIP part of fix)
Side note: considering what the patch fixes, I don't actually think we can undo it. But there's a possibility that we've actually uncovered an old bug that just was very unusual before, because the FOLL_ANON code was much harder to trigger before that patch (it only triggered if you didn't have a page table fully set up - now it triggers for even just a single missing page rather than a whole page table).
And I do see something pretty iffy in the logic for whether it can use FOLL_ANON or not - it had a nonsensical test for VM_LOCKED (which makes no sense at all), and it had never been taught about the ->nopfn() way of filling page tables.
So assuming it's not the XIP fix, you could try this patch instead. It replaces the (insane) use of VM_LOCKED with VM_SHARED (which is a lot more meaningful for the case of ZERO_PAGE, but strictly speaking probably doesn't matter either), and it teaches it about the fact that non-anonymous pages can be populated not just with the "->fault" handler, but with "->nopfn" too.
I really don't think it's due to this (nobody sane really uses '->nopfn'), but if the XIP disabling patch doesn't make a difference, give it a try.
(Most of the patch is obviously the fact that I moved the conditionals into a helper inline function to make the dang thing more readable). The actual change is trivial.
Linus
--- mm/memory.c | 12 ++++++++++-- 1 files changed, 10 insertions(+), 2 deletions(-)
diff --git a/mm/memory.c b/mm/memory.c index 9aefaae..8c5675f 100644 --- a/mm/memory.c +++ b/mm/memory.c @@ -1045,6 +1045,15 @@ no_page_table: return page; } +/* Can we do the FOLL_ANON optimization? */ +static inline int use_zero_page(struct vm_area_struct *vma) +{ + if (vma->vm_flags & VM_SHARED) + return 0; + return !vma->vm_ops || + (!vma->vm_ops->fault && !vma->vm_ops->nopfn); +} + int get_user_pages(struct task_struct *tsk, struct mm_struct *mm, unsigned long start, int len, int write, int force, struct page **pages, struct vm_area_struct **vmas) @@ -1119,8 +1128,7 @@ int get_user_pages(struct task_struct *tsk, struct mm_struct *mm, foll_flags = FOLL_TOUCH; if (pages) foll_flags |= FOLL_GET; - if (!write && !(vma->vm_flags & VM_LOCKED) && - (!vma->vm_ops || !vma->vm_ops->fault)) + if (!write && use_zero_page(vma)) foll_flags |= FOLL_ANON; do {
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