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

On Mon, Jun 03, 2019 at 09:16:07AM +0200, Michal Hocko wrote:

On Fri 31-05-19 23:34:07, Minchan Kim wrote:

quoted

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

quoted

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

quoted

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

quoted

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

quoted

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.

So what? In other words why should it be restricted solely based on the
map count. I can see a reason to restrict based on the access
permissions because we do not want to simplify all sorts of side channel
attacks but memory reclaim is capable of reclaiming shared pages and so
far I haven't heard any sound argument why madvise should skip those.
Again if there are any reasons, then document them in the changelog.

I think it makes sense. It could be explained, but it also follows
established madvise semantics, and I'm not sure it's necessarily
Minchan's job to re-iterate those.

Sharing isn't exactly transparent to userspace. The kernel does COW,
ksm etc. When you madvise, you can really only speak for your own
reference to that memory - "*I* am not using this."

This is in line with other madvise calls: MADV_DONTNEED clears the
local page table entries and drops the corresponding references, so
shared pages won't get freed. MADV_FREE clears the pte dirty bit and
also has explicit mapcount checks before clearing PG_dirty, so again
shared pages don't get freed.