Re: [PATCH v7 11/12] zsmalloc: page migration support

From: Dan Streetman <ddstreet@ieee.org>
To: Minchan Kim <minchan@kernel.org>
Cc: Chulmin Kim <cmlaika.kim@samsung.com>,
	Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Linux-MM <linux-mm@kvack.org>,
	Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Subject: Re: [PATCH v7 11/12] zsmalloc: page migration support
Date: Thu, 26 Jan 2017 12:04:03 -0500	[thread overview]
Message-ID: <CALZtONBRK10XwG7GkjSwsyGWw=X6LSjtNtPjJeZtMp671E5MOQ@mail.gmail.com> (raw)
In-Reply-To: <20170125052614.GB18289@bbox>

On Wed, Jan 25, 2017 at 12:26 AM, Minchan Kim <minchan@kernel.org> wrote:
> On Tue, Jan 24, 2017 at 11:06:51PM -0500, Chulmin Kim wrote:
>> On 01/23/2017 12:40 AM, Minchan Kim wrote:
>> >On Mon, Jan 23, 2017 at 02:30:56PM +0900, Sergey Senozhatsky wrote:
>> >>On (01/23/17 14:22), Minchan Kim wrote:
>> >>[..]
>> >>>>Anyway, I will let you know the situation when it gets more clear.
>> >>>
>> >>>Yeb, Thanks.
>> >>>
>> >>>Perhaps, did you tried flush page before the writing?
>> >>>I think arm64 have no d-cache alising problem but worth to try it.
>> >>>Who knows :)
>> >>
>> >>I thought that flush_dcache_page() is only for cases when we write
>> >>to page (store that makes pages dirty), isn't it?
>> >
>> >I think we need both because to see recent stores done by the user.
>> >I'm not sure it should be done by block device driver rather than
>> >page cache. Anyway, brd added it so worth to try it, I thought. :)
>> >
>>
>> Thanks for the suggestion!
>> It might be helpful
>> though proving it is not easy as the problem appears rarely.
>>
>> Have you thought about
>> zram swap or zswap dealing with self modifying code pages (ex. JIT)?
>> (arm64 may have i-cache aliasing problem)
>
> It can happen, I think, although I don't know how arm64 handles it.
>
>>
>> If it is problematic,
>> especiallly zswap (without flush_dcache_page in zswap_frontswap_load()) may
>> provide the corrupted data
>> and even swap out (compressing) may see the corrupted data sooner or later,
>> i guess.
>
> try_to_unmap_one calls flush_cache_page which I hope to handle swap-out side
> but for swap-in, I think zswap need flushing logic because it's first
> touch of the user buffer so it's his resposibility.

Hmm, I don't think zswap needs to, because all the cache aliases were
flushed when the page was written out.  After that, any access to the
page will cause a fault, and the fault will cause the page to be read
back in (via zswap).  I don't see how the page could be cached at any
time between the swap write-out and swap read-in, so there should be
no need to flush any caches when it's read back in; am I missing
something?

>
> Thanks.
>
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