From: Sergey Senozhatsky <email@example.com> To: Minchan Kim <firstname.lastname@example.org> Cc: Sergey Senozhatsky <email@example.com>, Andrew Morton <firstname.lastname@example.org>, email@example.com, Joonsoo Kim <firstname.lastname@example.org>, Sergey Senozhatsky <email@example.com>, kernel-team <firstname.lastname@example.org> 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
next prev parent 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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