Re: [RFC 1/2] deactive invalidated pages

[Minchan Kim <minchan.kim@gmail.com>]

On Tue, Nov 23, 2010 at 4:53 PM, Andrew Morton
<akpm@linux-foundation.org> wrote:
> On Tue, 23 Nov 2010 16:44:50 +0900 Minchan Kim <minchan.kim@gmail.com> wrote:
>
>> On Tue, Nov 23, 2010 at 4:15 PM, Andrew Morton
>> <akpm@linux-foundation.org> wrote:
>> > On Tue, 23 Nov 2010 15:05:39 +0900 Minchan Kim <minchan.kim@gmail.com> wrote:
>> >
>> >> On Tue, Nov 23, 2010 at 2:48 PM, Andrew Morton
>> >> >> > move it to the head of the LRU anyway. __But given that the user has
>> >> >>
>> >> >> Why does it move into head of LRU?
>> >> >> If the page which isn't mapped doesn't have PG_referenced, it would be
>> >> >> reclaimed.
>> >> >
>> >> > If it's dirty or under writeback it can't be reclaimed!
>> >>
>> >> I see your point. And it's why I add it to head of inactive list.
>> >
>> > But that *guarantees* that the page will get a full trip around the
>> > inactive list. __And this will guarantee that potentially useful pages
>> > are reclaimed before the pages which we *know* the user doesn't want!
>> > Bad!
>> >
>> > Whereas if we queue it to the tail, it will only get that full trip if
>> > reclaim happens to run before the page is cleaned. __And we just agreed
>> > that reclaim isn't likely to run immediately, because pages are being
>> > freed.
>> >
>> > So we face a choice between guaranteed eviction of potentially-useful
>> > pages (which are very expensive to reestablish) versus a *possible*
>> > need to move an unreclaimable page to the head of the LRU, which is
>> > cheap.
>>
>> How about flagging SetPageReclaim when we add it to head of inactive?
>> If page write is complete, end_page_writeback would move it to tail of
>> inactive.
>
> ooh, that sounds clever.  We'd want to do that for both PageDirty() and
> for PageWriteback() pages.
>
> But if we do it for PageDirty() pages, we'd need to clear PageReclaim()
> if someone reuses the page for some reason.  We'll end up with pages
> all over the place which have PageReclaim set.  I guess we could clear
> PageReclaim() in mark_page_accessed(), but that's hardly going to give
> us full coverage.
>
> hmm.  Maybe just do it for PageWriteback pages.  Then userspace can do
>
>        sync_file_range(SYNC_FILE_RANGE_WRITE);
>        fadvise(DONTNEED);
>
> and all those pages which now have PageWriteback set will also get
> PageReclaim set.
>
> But we'd need to avoid races against end_io when setting PageReclaim
> against the PageWriteback pages - if the interrupt happens while we're
> setting PageReclaim, it will end up being incorrectly set.
>

Okay. I will see it and resend new version.
Thanks for good comment, Andrew.



-- 
Kind regards,
Minchan Kim

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