On Thu, Apr 04 2013 at 9:36am -0400, Mikulas Patocka <mpatocka@xxxxxxxxxx> wrote: > > > On Thu, 4 Apr 2013, Mike Snitzer wrote: > > > On Thu, Apr 04 2013 at 8:55am -0400, > > Mikulas Patocka <mpatocka@xxxxxxxxxx> wrote: > > > > > > > > > > > On Thu, 4 Apr 2013, Mike Snitzer wrote: > > > > > > > I'll take a look at fixing this by deferring the scsi_dh switch until > > > > resume. This fix would assume multipath-tools is _not_ doing a noflush > > > > suspend/resume when it is switching the scsi_dh. > > > > > > > > Mike > > > > > > This won't work because scsi_dh_attach allocates memory and you can't > > > allocate memory when something is suspended. > > > > Ah yeah, scsi_dh->attach allocates memory for scsi_dh_data. But > > couldn't those scsi_dh_* attach allocations be switched from GFP_KERNEL > > to GFP_NOIO? > > Yes and no. > > GFP_NOIO allocations don't issue any IO, so they have higher possibility > of failure - if the memory is full of user space pages or dirty file pages > and there are no clean cache pages, then GFP_NOIO allocation can't make > any progress and fails. GFP_KERNEL allocation could swap out some pages > and succeed. > > On the other hand, kernel developers use GFP_NOIO allocations and assume > that they don't fail. And they don't fail most of the time. Although I > have seen cases where it failed and caused trouble - when allocating > inodes with GFP_NOIO under high memory stress. (the correct solution would > be to allocate the inode with GFP_KERNEL earlier, when we are not holding > any filesystem lock). > > >From the point of formal correctness, relying on GFP_NOIO is wrong, but if > the allocated space is small and it happens infrequently (only on > activation), you can likely get away with it without being caught. I'm suggesting that switching the scsi_dh is not something that will be done on a system that is suffering from serious memory contention. But I think we need to get back to analyzing the scsi_dh change you mentioned before with tracking counts, etc. -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
