linux-kernel - Re: [PATCH] mm,memory_failure: Always pin the page in madvise_inject

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Date:   Tue, 8 Dec 2020 02:35:14 +0000
From:   HORIGUCHI NAOYA(堀口　直也) 
        <naoya.horiguchi@....com>
To:     Oscar Salvador <osalvador@...e.de>
CC:     "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "n-horiguchi@...jp.nec.com" <n-horiguchi@...jp.nec.com>,
        "vbabka@...e.cz" <vbabka@...e.cz>,
        "dan.j.williams@...el.com" <dan.j.williams@...el.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm,memory_failure: Always pin the page in
 madvise_inject_error

On Mon, Dec 07, 2020 at 10:48:18AM +0100, Oscar Salvador wrote:
> madvise_inject_error() uses get_user_pages_fast to translate the
> address we specified to a page.
> After [1], we drop the extra reference count for memory_failure() path.
> That commit says that memory_failure wanted to keep the pin in order
> to take the page out of circulation.
> 
> The truth is that we need to keep the page pinned, otherwise the
> page might be re-used after the put_page() and we can end up messing
> with someone else's memory.
> 
> E.g:
> 
> CPU0
> process X					CPU1
>  madvise_inject_error
>   get_user_pages
>    put_page
> 					page gets reclaimed
> 					process Y allocates the page
>   memory_failure
>    // We mess with process Y memory
> 
> madvise() is meant to operate on a self address space, so messing with
> pages that do not belong to us seems the wrong thing to do.
> To avoid that, let us keep the page pinned for memory_failure as well.
> 
> Pages for DAX mappings will release this extra refcount in
> memory_failure_dev_pagemap.
> 
> [1] ("23e7b5c2e271: mm, madvise_inject_error:
>       Let memory_failure() optionally take a page reference")
> 
> Signed-off-by: Oscar Salvador <osalvador@...e.de>
> Suggested-by: Vlastimil Babka <vbabka@...e.cz>
> Fixes: 23e7b5c2e271 ("mm, madvise_inject_error: Let memory_failure() optionally take a page reference")

Thank you very much.

Acked-by: Naoya Horiguchi <naoya.horiguchi@....com>