linux-kernel - Re: [PATCH 2/2] zram: do not count duplicated pages as compressed

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Date:   Tue, 16 May 2017 16:36:17 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Minchan Kim <minchan@...nel.org>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, Joonsoo Kim <iamjoonsoo.kim@....com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        kernel-team <kernel-team@....com>
Subject: Re: [PATCH 2/2] zram: do not count duplicated pages as compressed

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