Re: [PATCH v2 01/13] mm: support madvise(MADV_FREE)

From: Andy Lutomirski
Date: Wed Nov 04 2015 - 19:43:44 EST

On Wed, Nov 4, 2015 at 4:13 PM, Minchan Kim <minchan@xxxxxxxxxx> wrote:
> On Tue, Nov 03, 2015 at 07:41:35PM -0800, Andy Lutomirski wrote:
>> On Nov 3, 2015 5:30 PM, "Minchan Kim" <minchan@xxxxxxxxxx> wrote:
>> >
>> > Linux doesn't have an ability to free pages lazy while other OS already
>> > have been supported that named by madvise(MADV_FREE).
>> >
>> > The gain is clear that kernel can discard freed pages rather than swapping
>> > out or OOM if memory pressure happens.
>> >
>> > Without memory pressure, freed pages would be reused by userspace without
>> > another additional overhead(ex, page fault + allocation + zeroing).
>> >
>> [...]
>> >
>> > How it works:
>> >
>> > When madvise syscall is called, VM clears dirty bit of ptes of the range.
>> > If memory pressure happens, VM checks dirty bit of page table and if it
>> > found still "clean", it means it's a "lazyfree pages" so VM could discard
>> > the page instead of swapping out. Once there was store operation for the
>> > page before VM peek a page to reclaim, dirty bit is set so VM can swap out
>> > the page instead of discarding.
>> What happens if you MADV_FREE something that's MAP_SHARED or isn't
>> ordinary anonymous memory? There's a long history of MADV_DONTNEED on
>> such mappings causing exploitable problems, and I think it would be
>> nice if MADV_FREE were obviously safe.
> It filter out VM_LOCKED|VM_HUGETLB|VM_PFNMAP and file-backed vma and MAP_SHARED
> with vma_is_anonymous.
>> Does this set the write protect bit?
> No.
>> What happens on architectures without hardware dirty tracking? For
>> that matter, even on architecture with hardware dirty tracking, what
>> happens in multithreaded processes that have the dirty TLB state
>> cached in a different CPU's TLB?
>> Using the dirty bit for these semantics scares me. This API creates a
>> page that can have visible nonzero contents and then can
>> asynchronously and magically zero itself thereafter. That makes me
>> nervous. Could we use the accessed bit instead? Then the observable
> Access bit is used by aging algorithm for reclaim. In addition,
> we have supported clear_refs feacture.
> IOW, it could be reset anytime so it's hard to use marker for
> lazy freeing at the moment.

That's unfortunate. I think that the ABI would be much nicer if it
used the accessed bit.

In any case, shouldn't the aging algorithm be irrelevant here? A
MADV_FREE page that isn't accessed can be discarded, whereas we could
hopefully just say that a MADV_FREE page that is accessed gets moved
to whatever list holds recently accessed pages and also stops being a
candidate for discarding due to MADV_FREE?

>> > + if (!PageDirty(page) && (flags & TTU_FREE)) {
>> > + /* It's a freeable page by MADV_FREE */
>> > + dec_mm_counter(mm, MM_ANONPAGES);
>> > + goto discard;
>> > + }
>> Does something clear TTU_FREE the next time the page gets marked clean?
> Sorry, I don't understand. Could you elaborate it more?

I don't fully understand how TTU_FREE ends up being set here, but, if
the page is dirtied by user code and then cleaned later by the kernel,
what prevents TTU_FREE from being incorrectly set here?

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