On Wed 08-03-17 20:23:37, Tetsuo Handa wrote: > On 2017/03/08 0:48, Michal Hocko wrote: > > From: Michal Hocko <mhocko@xxxxxxxx> > > > > KM_MAYFAIL didn't have any suitable GFP_FOO counterpart until recently > > so it relied on the default page allocator behavior for the given set > > of flags. This means that small allocations actually never failed. > > > > Now that we have __GFP_RETRY_MAYFAIL flag which works independently on the > > allocation request size we can map KM_MAYFAIL to it. The allocator will > > try as hard as it can to fulfill the request but fails eventually if > > the progress cannot be made. > > > > Cc: Darrick J. Wong <darrick.wong@xxxxxxxxxx> > > Signed-off-by: Michal Hocko <mhocko@xxxxxxxx> > > --- > > fs/xfs/kmem.h | 10 ++++++++++ > > 1 file changed, 10 insertions(+) > > > > diff --git a/fs/xfs/kmem.h b/fs/xfs/kmem.h > > index ae08cfd9552a..ac80a4855c83 100644 > > --- a/fs/xfs/kmem.h > > +++ b/fs/xfs/kmem.h > > @@ -54,6 +54,16 @@ kmem_flags_convert(xfs_km_flags_t flags) > > lflags &= ~__GFP_FS; > > } > > > > + /* > > + * Default page/slab allocator behavior is to retry for ever > > + * for small allocations. We can override this behavior by using > > + * __GFP_RETRY_MAYFAIL which will tell the allocator to retry as long > > + * as it is feasible but rather fail than retry for ever for all > > + * request sizes. > > + */ > > + if (flags & KM_MAYFAIL) > > + lflags |= __GFP_RETRY_MAYFAIL; > > I don't see advantages of supporting both __GFP_NORETRY and __GFP_RETRY_MAYFAIL. > kmem_flags_convert() can always set __GFP_NORETRY because the callers use > opencoded __GFP_NOFAIL loop (with possible allocation lockup warning) unless > KM_MAYFAIL is set. The behavior would be different (e.g. the OOM killer handling). [...] > line, which is likely always true); but this is off-topic for this thread. yes [...] > where both __GFP_NORETRY and __GFP_RETRY_MAYFAIL are checked after > direct reclaim and compaction failed. __GFP_RETRY_MAYFAIL optimistically > retries based on one of should_reclaim_retry() or should_compact_retry() > or read_mems_allowed_retry() returns true or mutex_trylock(&oom_lock) in > __alloc_pages_may_oom() returns 0. If !__GFP_FS allocation requests are > holding oom_lock each other, __GFP_RETRY_MAYFAIL allocation requests (which > are likely !__GFP_FS allocation requests due to __GFP_FS allocation requests > being blocked on direct reclaim) can be blocked for uncontrollable duration > without making progress. It seems to me that the difference between > __GFP_NORETRY and __GFP_RETRY_MAYFAIL is not useful. Rather, the caller can > set __GFP_NORETRY and retry with any control (e.g. set __GFP_HIGH upon first > timeout, give up upon second timeout). You are drown in implementation details here. Try to step back and think about the high level semantic I would like to achieve - which is essentially a middle ground between __GFP_NORETRY which doesn't retry and __GFP_NOFAIL to retry for ever. There are users who could benefit from such a semantic I believe (the most prominent example is kvmalloc which has different modes of how hard to try kmalloc before giving up and falling back to vmalloc).. -- Michal Hocko SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
