Re: [RFCv2 1/6] mm: introduce MADV_COLD

On Fri, May 31, 2019 at 04:03:32PM +0200, Michal Hocko wrote:

On Fri 31-05-19 22:39:04, Minchan Kim wrote:

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On Fri, May 31, 2019 at 10:47:52AM +0200, Michal Hocko wrote:

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On Fri 31-05-19 15:43:08, Minchan Kim wrote:

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When a process expects no accesses to a certain memory range, it could
give a hint to kernel that the pages can be reclaimed when memory pressure
happens but data should be preserved for future use.  This could reduce
workingset eviction so it ends up increasing performance.

This patch introduces the new MADV_COLD hint to madvise(2) syscall.
MADV_COLD can be used by a process to mark a memory range as not expected
to be used in the near future. The hint can help kernel in deciding which
pages to evict early during memory pressure.

Internally, it works via deactivating pages from active list to inactive's
head if the page is private because inactive list could be full of
used-once pages which are first candidate for the reclaiming and that's a
reason why MADV_FREE move pages to head of inactive LRU list. Therefore,
if the memory pressure happens, they will be reclaimed earlier than other
active pages unless there is no access until the time.

[I am intentionally not looking at the implementation because below
points should be clear from the changelog - sorry about nagging ;)]

What kind of pages can be deactivated? Anonymous/File backed.
Private/shared? If shared, are there any restrictions?

Both file and private pages could be deactived from each active LRU
to each inactive LRU if the page has one map_count. In other words,

    if (page_mapcount(page) <= 1)
        deactivate_page(page);

Why do we restrict to pages that are single mapped?

Because page table in one of process shared the page would have access bit
so finally we couldn't reclaim the page. The more process it is shared,
the more fail to reclaim.

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Are there any restrictions on mappings? E.g. what would be an effect of
this operation on hugetlbfs mapping?

VM_LOCKED|VM_HUGETLB|VM_PFNMAP vma will be skipped like MADV_FREE|DONTNEED

OK documenting that this is restricted to the same vmas as MADV_FREE|DONTNEED
is really useful to mention.

Sure.

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Also you are talking about inactive LRU but what kind of LRU is that? Is
it the anonymous LRU? If yes, don't we have the same problem as with the

active file page -> inactive file LRU
active anon page -> inacdtive anon LRU

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early MADV_FREE implementation when enough page cache causes that
deactivated anonymous memory doesn't get reclaimed anytime soon. Or
worse never when there is no swap available?

I think MADV_COLD is a little bit different symantic with MAVD_FREE.
MADV_FREE means it's okay to discard when the memory pressure because
the content of the page is *garbage*. Furthemore, freeing such pages is
almost zero overhead since we don't need to swap out and access
afterward causes minor fault. Thus, it would make sense to put those
freeable pages in inactive file LRU to compete other used-once pages.

However, MADV_COLD doesn't means it's a garbage and freeing requires
swap out/swap in afterward. So, it would be better to move inactive
anon's LRU list, not file LRU. Furthermore, it would avoid unnecessary
scanning of those cold anonymous if system doesn't have a swap device.

Please document this, if this is really a desirable semantic because
then you have the same set of problems as we've had with the early
MADV_FREE implementation mentioned above.

IIRC, the problem of MADV_FREE was that we couldn't discard freeable
pages because VM never scan anonymous LRU with swapless system.
However, it's not the our case because we should reclaim them, not
discarding.

I will include it in the description.

Thanks.