linux-kernel - Re: interaction of MADV_PAGEOUT with CoW anonymous mappings?

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Date:   Thu, 12 Mar 2020 14:41:07 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Minchan Kim <minchan@...nel.org>, Michal Hocko <mhocko@...nel.org>
Cc:     Jann Horn <jannh@...gle.com>, Linux-MM <linux-mm@...ck.org>,
        kernel list <linux-kernel@...r.kernel.org>,
        Daniel Colascione <dancol@...gle.com>,
        "Joel Fernandes (Google)" <joel@...lfernandes.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: interaction of MADV_PAGEOUT with CoW anonymous mappings?

One other fun thing.  I have a "victim" thread sitting in a loop doing:

	sleep(1)
	memcpy(&garbage, buffer, sz);

The "attacker" is doing

	madvise(buffer, sz, MADV_PAGEOUT);

in a loop.  That, oddly enough doesn't cause the victim to page fault.
But, if I do:

	memcpy(&garbage, buffer, sz);
	madvise(buffer, sz, MADV_PAGEOUT);

It *does* cause the memory to get paged out.  The MADV_PAGEOUT code
actually has a !pte_present() check.  It will punt on a PTE if it sees
it.  In other words, if a page is in the swap cache but not mapped by a
pte_present() PTE, MADV_PAGEOUT won't touch it.

Shouldn't MADV_PAGEOUT be able to find and reclaim those pages?  Patch
attached.

View attachment "madv-pageout-find-swap-cache.patch" of type "text/x-patch" (1630 bytes)