On Mon, Sep 26, 2011 at 05:40:52PM +0530, kautuk.c @samsung.com wrote:

On Mon, Sep 26, 2011 at 4:59 PM, Johannes Weiner [off-list ref] wrote:

quoted

On Sun, Sep 25, 2011 at 04:29:40PM +0530, Kautuk Consul wrote:

quoted

write_scan_unavictable_node checks the value req returned by
strict_strtoul and returns 1 if req is 0.

However, when strict_strtoul returns 0, it means successful conversion
of buf to unsigned long.

Due to this, the function was not proceeding to scan the zones for
unevictable pages even though we write a valid value to the
scan_unevictable_pages sys file.

Given that there is not a real reason for this knob (anymore) and that
it apparently never really worked since the day it was introduced, how
about we just drop all that code instead?

       Hannes

---
From: Johannes Weiner <redacted>
Subject: mm: remove sysctl to manually rescue unevictable pages

At one point, anonymous pages were supposed to go on the unevictable
list when no swap space was configured, and the idea was to manually
rescue those pages after adding swap and making them evictable again.
But nowadays, swap-backed pages on the anon LRU list are not scanned
without available swap space anyway, so there is no point in moving
them to a separate list anymore.

Is this code only for anonymous pages ?
It seems to look at all pages in the zone both file as well as anon.

The code scans both, but the usecase I described was moving swapbacked
pages from the unevictable list after configuring swap space.

quoted

The manual rescue could also be used in case pages were stranded on
the unevictable list due to race conditions.  But the code has been
around for a while now and newly discovered bugs should be properly
reported and dealt with instead of relying on such a manual fixup.

What you say seems to be all right for anon pages, but what about file
pages ?
I'm not sure about how this could happen, but what if some file-system caused
a file cache page to be set to evictable or reclaimable without
actually removing
that page from the unevictable list ?

Currently, unevictable file pages come from ramfs, shmem, and mlock.
ramfs never makes them evictable.  shmem, after making an inode
evictable again, scans the that inode's address_space and rescues the
pages it finds.  munlock synchroneously rescues the pages vma's range.

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