On Mon, Nov 27 2017, Mike Snitzer wrote: > On Thu, Nov 23 2017 at 5:52pm -0500, > NeilBrown <neilb@xxxxxxxx> wrote: > >> >> When we use bio_clone_bioset() to split off the front part of a bio >> and chain the two together and submit the remainder to >> generic_make_request(), it is important that the newly allocated >> bio is used as the head to be processed immediately, and the original >> bio gets "bio_advance()"d and sent to generic_make_request() as the >> remainder. >> >> If the newly allocated bio is used as the remainder, and if it then >> needs to be split again, then the next bio_clone_bioset() call will >> be made while holding a reference a bio (result of the first clone) >> from the same bioset. This can potentially exhaust the bioset mempool >> and result in a memory allocation deadlock. >> >> So the result of the bio_clone_bioset() must be attached to the new >> dm_io struct, and the original must be resubmitted. The current code >> is backwards. >> >> Note that there is no race caused by reassigning cio.io->bio after already >> calling __map_bio(). This bio will only be dereferenced again after >> dec_pending() has found io->io_count to be zero, and this cannot happen >> before the dec_pending() call at the end of __split_and_process_bio(). >> >> Reported-by: Mikulas Patocka <mpatocka@xxxxxxxxxx> >> Signed-off-by: NeilBrown <neilb@xxxxxxxx> >> --- >> >> Hi, >> I think this should resolve the problem Mikulas noticed that the >> bios form a deep chain instead of a wide tree. > > I'm inclined to just fold this into the original commit. > I'd update that header to make mention of the details captured in this > header. > > Would you be OK with that? Perfectly OK with that. Thanks for asking. NeilBrown
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