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From: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
To: Minchan Kim <minchan@kernel.org>
Cc: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	linux-kernel@vger.kernel.org,
	Joonsoo Kim <iamjoonsoo.kim@lge.com>,
	Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
	kernel-team <kernel-team@lge.com>
Subject: Re: [PATCH 2/2] zram: do not count duplicated pages as compressed
Date: Tue, 16 May 2017 16:36:17 +0900	[thread overview]
Message-ID: <20170516073616.GB767@jagdpanzerIV.localdomain> (raw)
In-Reply-To: <20170516071644.GA6224@bbox>

On (05/16/17 16:16), Minchan Kim wrote:
> > but would this be correct? the data is not valid - we failed to store
> > the valid one. but instead we assure application that read()/swapin/etc.,
> > depending on the usage scenario, is successful (even though the data is
> > not what application really expects to see), application tries to use the
> > data from that page and probably crashes (dunno, for example page contained
> > hash tables with pointers that are not valid anymore, etc. etc.).
> > 
> > I'm not optimistic about stale data reads; it basically will look like
> > data corruption to the application.
> 
> Hmm, I don't understand what you say.
> My point is zram_free_page should be done only if whoe write operation
> is successful.
> With you change, following situation can happens.
> 
> write block 4, 'all A' -> success
> read  block 4, 'all A' verified -> Good
> write block 4, 'all B' -> but failed with ENOMEM
> read  block 4  expected 'all A' but 'all 0' -> Oops

yes. 'all A' in #4 can be incorrect. zram can be used as a block device
with a file system, and pid that does write op not necessarily does read
op later. it can be a completely different application. e.g. compilation,
or anything else.

suppose PID A does

wr block 1       all a
wr block 2       all a + 1
wr block 3       all a + 2
wr block 4       all a + 3

now PID A does

wr block 1       all m
wr block 2       all m + 1
wr block 3       all m + 2
wr block 4       failed. block still has 'all a + 3'.
exit

another application, PID C, reads in the file and tries to do
something sane with it

rd block 1       all m
rd block 2       all m + 1
rd block 3       all m + 3
rd block 4       all a + 3      << this is dangerous. we should return
                                   error from read() here; not stale data.


what we can return now is a `partially updated' data, with some new
and some stale pages. this is quite unlikely to end up anywhere good.
am I wrong?

why does `rd block 4' in your case causes Oops? as a worst case scenario?
application does not expect page to be 'all A' at this point. pages are
likely to belong to some mappings/files/etc., and there is likely a data
dependency between them, dunno C++ objects that span across pages or
JPEG images, etc. so returning "new data   new data   stale data" is a bit
fishy.

	-ss

  reply	other threads:[~2017-05-16  7:36 UTC|newest]

Thread overview: 15+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-05-15  7:41 [PATCH 1/2] zram: count same page write as page_stored Minchan Kim
2017-05-15  7:41 ` [PATCH 2/2] zram: do not count duplicated pages as compressed Minchan Kim
2017-05-16  1:30   ` Sergey Senozhatsky
2017-05-16  1:59     ` Minchan Kim
2017-05-16  2:36       ` Sergey Senozhatsky
2017-05-16  5:26         ` Minchan Kim
2017-05-16  5:45           ` Sergey Senozhatsky
2017-05-16  7:16             ` Minchan Kim
2017-05-16  7:36               ` Sergey Senozhatsky [this message]
2017-05-17  8:32                 ` Minchan Kim
2017-05-17  9:14                   ` Sergey Senozhatsky
2017-05-18  4:53                     ` Minchan Kim
2017-05-21  7:04                   ` Christoph Hellwig
2017-05-21  7:15                     ` Minchan Kim
2017-05-16  1:11 ` [PATCH 1/2] zram: count same page write as page_stored Sergey Senozhatsky

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