linux-kernel - Re: [PATCH] mlock: revert the optimization for dirtying pages and triggering writeback.

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Message-ID: <AANLkTik1dt1Q9TA+JmdvkuOqmt5LB2iZ1X2B5GbBFx1+@mail.gmail.com>
Date:	Sun, 30 Jan 2011 02:26:27 -0800
From:	Michel Lespinasse <walken@...gle.com>
To:	Tao Ma <tm@....ma>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] mlock: revert the optimization for dirtying pages and
 triggering writeback.

On Sat, Jan 29, 2011 at 11:15 PM, Tao Ma <tm@....ma> wrote:
>        buf = mmap(NULL, file_len, PROT_WRITE, MAP_SHARED, fd, 0);
>        if (buf == MAP_FAILED) {
>                perror("mmap");
>                goto out;
>        }
>
>        if (mlock(buf, file_len) < 0) {
>                perror("mlock");
>                goto out;
>        }

Thanks Tao for tracing this to an individual change. I can reproduce
this on my system. The issue is that the file is mapped without the
PROT_READ permission, so mlock can't fault in the pages. Up to 2.6.37
this worked because mlock was using a write.

The test case does show there was a behavior change; however it's not
clear to me that the tested behavior is valid.

I can see two possible resolutions:

1- do nothing, if we can agree that the test case is invalid

2- restore the previous behavior for writable, non-readable, shared
mappings while preserving the optimization for read/write shared
mappings. The test would then look like:
        if ((vma->vm_flags & VM_WRITE) && (vma->vm_flags & (VM_READ |
VM_SHARED)) != VM_SHARED)
                gup_flags |= FOLL_WRITE;

-- 
Michel "Walken" Lespinasse
A program is never fully debugged until the last user dies.
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