On Wed, Aug 05, 2015 at 09:15:57PM -0700, Hugh Dickins wrote:
> Hi Kirill,
>
> I had a nasty thought this morning.
Tough day.
I'm trying to wrap my head around this mail and not sure if I succeed
much. :-|
> Andrew had prodded me gently to re-examine my concerns with your
> page-flags rework in mmotm. I still dislike the bloat (my mm/built-in.o
> text goes up from 478513 to 490183 bytes on a non-DEBUG_VM build); but I
> was hoping to set that aside, to let us move forward.
>
> But looking into the bloat led me to what seems a more serious issue
> with it. I'd tacked a little function on to the end of mm/filemap.c:
>
> bool page_is_locked(struct page *page)
> {
> return !!PageLocked(page);
> }
>
> which came out as:
>
> 0000000000003a60 <page_is_locked>:
> 3a60: 48 8b 07 mov (%rdi),%rax
> 3a63: 55 push %rbp
> 3a64: 48 89 e5 mov %rsp,%rbp
>
> [instructions above same as without your patches; those below added by them]
>
> 3a67: f6 c4 80 test $0x80,%ah
> 3a6a: 74 10 je 3a7c <page_is_locked+0x1c>
> 3a6c: 48 8b 47 30 mov 0x30(%rdi),%rax
> 3a70: 48 8b 17 mov (%rdi),%rdx
> 3a73: 80 e6 80 and $0x80,%dh
> 3a76: 48 0f 44 c7 cmove %rdi,%rax
> 3a7a: eb 03 jmp 3a7f <page_is_locked+0x1f>
> 3a7c: 48 89 f8 mov %rdi,%rax
> 3a7f: 48 8b 00 mov (%rax),%rax
>
> [instructions above added by your patches; those below same as before]
>
> 3a82: 5d pop %rbp
> 3a83: 83 e0 01 and $0x1,%eax
> 3a86: c3 retq
>
> The "and $0x80,%dh" looked superfluous at first, but of course it isn't:
> it's from the smp_rmb() in David's 668f9abbd433 "mm: close PageTail race"
> (a later commit refactors compound_head() but doesn't change the story).
>
> And it's that race, or a worse race of that kind, that now worries me.
> Relying on smp_wmb() and smp_rmb() may be all that was needed in the
> case that David was fixing; and (I dare not look at them to audit!)
> all uses of compound_head() in our current v4.2-rc tree may well be
> safe, for this or that contingent reason in each place that it's used.
>
> But there is no locking within compound_head(page) to make it safe
> everywhere, yet your page-flags rework is changing a large number
> of PageWhatever()s and SetPageWhatever()s and ClearPageWhatever()s
> now to do a hidden compound_head(page) beneath the covers.
>
> To be more specific: if preemption, or an interrupt, or entry to SMM
> mode, or whatever, delays this thread somewhere in that compound_head()
> sequence of instructions, how can we be sure that the "head" returned
> by compound_head() is good? We know the page was PageTail just before
> looking up page->first_page, and we know it was PageTail just after,
> but we don't know that it was PageTail throughout, and we don't know
> whether page->first_page is even a good page pointer, or something
> else from the private/ptl/slab_cache union.
That looks like a very valid worry to me. For current -mm tree.
But let's take my refcounting rework into picture.
One thing it simplifies is protection against splitting. Once you've got a
reference to a page, it cannot be split under you. It makes PageTail() and
->first_page stable for most callsites.
We can access the page's flags under ptl, without having reference the
page. And that's fine: ptl protects against splitting too.
Fast GUP also have a way to protect against split.
IIUC, the only potentially problematic callsites left are physical memory
scanners. This code requires audit. I'll do that.
Do I miss something else?
> Of course it would be very rare for it to go wrong; and most callsites
> will obviously be safe for this or that reason; though, sadly, none of
> them safe from holding a reference to the tail page in question, since
> its count is frozen at 0 and cannot be grabbed by get_page_unless_zero.
Do you mean that grabbing head page's ->_count is not enough to protect
against splitting and freeing tail page under you?
I know a patchset which solves this! ;)
> But I don't see how it can be safe to rely on compound_head() inside
> a general purpose page-flag function, that we're all accustomed to
> think of as a simple bitop, that can be applied without great care.
>
> Hugh
>
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--
Kirill A. Shutemov
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