Hi Ted, On Wed, Oct 30, 2019 at 11:04:37AM -0400, Theodore Y. Ts'o wrote: > On Wed, Oct 30, 2019 at 03:43:10PM +0530, Ritesh Harjani wrote: > > > > > > On 10/30/19 9:56 AM, Gao Xiang wrote: > > > Similar to [1] [2], it seems a trivial cleanup since > > > bio_alloc can handle memory allocation as mentioned in > > > fs/direct-io.c (also see fs/block_dev.c, fs/buffer.c, ..) > > > > > > > AFAIU, the reason is that, bio_alloc with __GFP_DIRECT_RECLAIM > > flags guarantees bio allocation under some given restrictions, > > as stated in fs/direct-io.c > > So here it is ok to not check for NULL value from bio_alloc. > > > > I think we can update above info too in your commit msg. > > Please also add a short comment in the code itself, so it's clear why > it's OK to skip the error check, and reference the comments for > bio_alloc_bioset(). This is the fairly subtle bit which makes this > change not obvious: OK, I will add short comments in code then, and tidy up later since it's not urgent (but I'm surprised that so many in-kernel code handles that, those also makes me misleaded before, but I think mempool back maybe better since the total efficient path is shorter compared with error handling path)... and I'd like to know the f2fs side as well :-) > > * When @bs is not NULL, if %__GFP_DIRECT_RECLAIM is set then bio_alloc will > * always be able to allocate a bio. This is due to the mempool guarantees. > * To make this work, callers must never allocate more than 1 bio at a time > * from this pool. Callers that need to allocate more than 1 bio must always > * submit the previously allocated bio for IO before attempting to allocate > * a new one. Failure to do so can cause deadlocks under memory pressure. > * > * Note that when running under generic_make_request() (i.e. any block > * driver), bios are not submitted until after you return - see the code in > * generic_make_request() that converts recursion into iteration, to prevent > * stack overflows. > * > * This would normally mean allocating multiple bios under > * generic_make_request() would be susceptible to deadlocks, but we have > * deadlock avoidance code that resubmits any blocked bios from a rescuer > * thread. > > Otherwise, someone else may not understand why it's safe to not check > the error return then submit cleanup patch to add the error checking > back. :-) Got it. Thanks, Gao Xiang > > - Ted >
