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

[Minchan Kim]

On Wed, Nov 04, 2015 at 04:42:37PM -0800, Andy Lutomirski wrote:
> 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?

I meant if we use access bit as indicator for lazy-freeing page,
we could discard valid page which is never hinted by MADV_FREE but
just doesn't mark access bit in page table by aging algorithm.

> 
> >>
> >> > +               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?

Kernel shouldn't make the page clean without writeback(ie, swapout)
if the page has valid data.

> 
> 
> --Andy
> 
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