Re: [PATCH 4/4] capture pages freed during direct reclaim for allocation by the reclaimer

[KOSAKI Motohiro]

> When a process enters direct reclaim it will expend effort identifying
> and releasing pages in the hope of obtaining a page.  However as these
> pages are released asynchronously there is every possibility that the
> pages will have been consumed by other allocators before the reclaimer
> gets a look in.  This is particularly problematic where the reclaimer is
> attempting to allocate a higher order page.  It is highly likely that
> a parallel allocation will consume lower order constituent pages as we
> release them preventing them coelescing into the higher order page the
> reclaimer desires.
> 
> This patch set attempts to address this for allocations above
> ALLOC_COSTLY_ORDER by temporarily collecting the pages we are releasing
> onto a local free list.  Instead of freeing them to the main buddy lists,
> pages are collected and coelesced on this per direct reclaimer free list.
> Pages which are freed by other processes are also considered, where they
> coelesce with a page already under capture they will be moved to the
> capture list.  When pressure has been applied to a zone we then consult
> the capture list and if there is an appropriatly sized page available
> it is taken immediatly and the remainder returned to the free pool.
> Capture is only enabled when the reclaimer's allocation order exceeds
> ALLOC_COSTLY_ORDER as free pages below this order should naturally occur
> in large numbers following regular reclaim.


Hi Andy,

I like almost part of your patch.
(at least, I can ack patch 1/4 - 3/4)

So, I worry about OOM risk.
Can you remember desired page size to capture list (or any other location)?
if possible, __capture_on_page can avoid to capture unnecessary pages.

So, if __capture_on_page() can make desired size page by buddy merging, 
it can free other pages on capture_list.

In worst case, shrink_zone() is called by very much process at the same time.
Then, if each process doesn't back few pages, very many pages doesn't be backed.



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