Re: [PATCH v3 01/17] mm: support madvise(MADV_FREE)

On Wed, Nov 11, 2015 at 8:32 PM, Minchan Kim [off-list ref] 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.

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.

I realize that this lends itself to an efficient implementation, but
it's certainly the case that the kernel *could* use the accessed bit
instead of the dirty bit to give more sensible user semantics, and the
semantics that rely on the dirty bit make me uncomfortable from an ABI
perspective.

I also think that the kernel should commit to either zeroing the page
or leaving it unchanged in response to MADV_FREE (even if the decision
of which to do is made later on).  I think that your patch series does
this, but only after a few of the patches are applied (the swap entry
freeing), and I think that it should be a real guaranteed part of the
semantics and maybe have a test case.

--Andy

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