linux-kernel - Re: [PATCH -v8 3/4] Enable the MS_ASYNC functionality in sys

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Message-ID: <4df4ef0c0801230926g39cd8d93vc705cf77f8164e55@mail.gmail.com>
Date:	Wed, 23 Jan 2008 20:26:20 +0300
From:	"Anton Salikhmetov" <salikhmetov@...il.com>
To:	"Linus Torvalds" <torvalds@...ux-foundation.org>
Cc:	linux-mm@...ck.org, jakob@...hought.net,
	linux-kernel@...r.kernel.org, valdis.kletnieks@...edu,
	riel@...hat.com, ksm@...dk, staubach@...hat.com,
	jesper.juhl@...il.com, a.p.zijlstra@...llo.nl,
	akpm@...ux-foundation.org, protasnb@...il.com, miklos@...redi.hu,
	r.e.wolff@...wizard.nl, hidave.darkstar@...il.com,
	hch@...radead.org
Subject: Re: [PATCH -v8 3/4] Enable the MS_ASYNC functionality in sys_msync()

2008/1/23, Linus Torvalds <torvalds@...ux-foundation.org>:
>
>
> On Wed, 23 Jan 2008, Anton Salikhmetov wrote:
> > +
> > +             if (pte_dirty(*pte) && pte_write(*pte)) {
>
> Not correct.
>
> You still need to check "pte_present()" before you can test any other
> bits. For a non-present pte, none of the other bits are defined, and for
> all we know there might be architectures out there that require them to
> be non-dirty.
>
> As it is, you just possibly randomly corrupted the pte.
>
> Yeah, on all architectures I know of, it the pte is clear, neither of
> those tests will trigger, so it just happens to work, but it's still
> wrong. And for a MAP_SHARED mapping, it should be either clear or valid,
> although I can imagine that we might do swap-cache entries for tmpfs or
> something (in which case trying to clear the write-enable bit would
> corrupt the swap entry!).
>
> So the bug might be hard or even impossible to trigger in practice, but
> it's still wrong.
>
> I realize that "page_mkclean_one()" doesn't do this very obviously, but
> it's actually there (it's just hidden in page_check_address()).
>
> Quite frankly, at this point I'm getting *very* tired of this series.
> Especially since you ignored me when I suggested you just revert the
> commit that removed the page table walking - and instead send in a buggy
> patch.
>
> Yes, the VM is hard. I agree. It's nasty. But exactly because it's nasty
> and subtle and horrid, I'm also very anal about it, and I get really
> nervous when somebody touches it without (a) knowing all the rules
> intimately and (b) listening to people who do.
>
> So here's even a patch to get you started. Do this:
>
>         git revert 204ec841fbea3e5138168edbc3a76d46747cc987
>
> and then use this appended patch on top of that as a starting point for
> something that compiles and *possibly* works.
>
> And no, I do *not* guarantee that this is right either! I have not tested
> it or thought about it a lot, and S390 tends to be odd about some of these
> things. In particular, I actually suspect that we should possibly do this
> the same way we do
>
>         ptep_clear_flush_young()
>
> except we would do "ptep_clear_flush_wrprotect()". So even though this is
> a revert plus a simple patch to make it compile again (we've changed how
> we do dirty bits), I think a patch like this needs testing and other
> people like Nick and Peter to ack it.

I'm very sorry for my bad code which can not pass LKML's review.

I reassigned the bug #2645 to default assignee, Andrew Morton, because
it seems that people start getting tired of my patch series.

Thanks for your support.

>
> Nick? Peter? Testing? Other comments?
>
>                 Linus
>
> ---
>  mm/msync.c |    9 ++++++---
>  1 files changed, 6 insertions(+), 3 deletions(-)
>
> diff --git a/mm/msync.c b/mm/msync.c
> index a30487f..9b0af8f 100644
> --- a/mm/msync.c
> +++ b/mm/msync.c
> @@ -32,6 +32,7 @@ static unsigned long msync_pte_range(struct vm_area_struct *vma, pmd_t *pmd,
>  again:
>         pte = pte_offset_map_lock(vma->vm_mm, pmd, addr, &ptl);
>         do {
> +               pte_t entry;
>                 struct page *page;
>
>                 if (progress >= 64) {
> @@ -47,9 +48,11 @@ again:
>                 page = vm_normal_page(vma, addr, *pte);
>                 if (!page)
>                         continue;
> -               if (ptep_clear_flush_dirty(vma, addr, pte) ||
> -                               page_test_and_clear_dirty(page))
> -                       ret += set_page_dirty(page);
> +               entry = ptep_clear_flush(vma, addr, pte);
> +               entry = pte_wrprotect(entry);
> +               set_pte_at(mm, address, pte, entry);
> +
> +               ret += 1;
>                 progress += 3;
>         } while (pte++, addr += PAGE_SIZE, addr != end);
>         pte_unmap_unlock(pte - 1, ptl);
>
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