Hi Sergey, On Tue, May 16, 2017 at 04:36:17PM +0900, Sergey Senozhatsky wrote: > 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. I thought more about it and start to confuse. :/ So, let's Cc linux-block, fs peoples. The question is that Is block device(esp, zram which is compressed ram block device) okay to return garbage when ongoing overwrite IO fails? O_DIRECT write 4 block "aaa.." -> success read 4 block "aaa.." -> success O_DIRECT write 4 block "bbb.." -> fail read 4 block "000..' -> it is okay? Hope to get an answer form experts. :)
