On Fri, 22 Jun 2018, Michal Hocko wrote: > On Fri 22-06-18 08:52:09, Mikulas Patocka wrote: > > > > > > On Fri, 22 Jun 2018, Michal Hocko wrote: > > > > > On Fri 22-06-18 11:01:51, Michal Hocko wrote: > > > > On Thu 21-06-18 21:17:24, Mikulas Patocka wrote: > > > [...] > > > > > What about this patch? If __GFP_NORETRY and __GFP_FS is not set (i.e. the > > > > > request comes from a block device driver or a filesystem), we should not > > > > > sleep. > > > > > > > > Why? How are you going to audit all the callers that the behavior makes > > > > sense and moreover how are you going to ensure that future usage will > > > > still make sense. The more subtle side effects gfp flags have the harder > > > > they are to maintain. > > > > > > So just as an excercise. Try to explain the above semantic to users. We > > > currently have the following. > > > > > > * __GFP_NORETRY: The VM implementation will try only very lightweight > > > * memory direct reclaim to get some memory under memory pressure (thus > > > * it can sleep). It will avoid disruptive actions like OOM killer. The > > > * caller must handle the failure which is quite likely to happen under > > > * heavy memory pressure. The flag is suitable when failure can easily be > > > * handled at small cost, such as reduced throughput > > > > > > * __GFP_FS can call down to the low-level FS. Clearing the flag avoids the > > > * allocator recursing into the filesystem which might already be holding > > > * locks. > > > > > > So how are you going to explain gfp & (__GFP_NORETRY | ~__GFP_FS)? What > > > is the actual semantic without explaining the whole reclaim or force > > > users to look into the code to understand that? What about GFP_NOIO | > > > __GFP_NORETRY? What does it mean to that "should not sleep". Do all > > > shrinkers have to follow that as well? > > > > My reasoning was that there is broken code that uses __GFP_NORETRY and > > assumes that it can't fail - so conditioning the change on !__GFP_FS would > > minimize the diruption to the broken code. > > > > Anyway - if you want to test only on __GFP_NORETRY (and fix those 16 > > broken cases that assume that __GFP_NORETRY can't fail), I'm OK with that. > > As I've already said, this is a subtle change which is really hard to > reason about. Throttling on congestion has its meaning and reason. Look > at why we are doing that in the first place. You cannot simply say this So - explain why is throttling needed. You support throttling, I don't, so you have to explain it :) > is ok based on your specific usecase. We do have means to achieve that. > It is explicit and thus it will be applied only where it makes sense. > You keep repeating that implicit behavior change for everybody is > better. I don't want to change it for everybody. I want to change it for block device drivers. I don't care what you do with non-block drivers. > I guess we will not agree on that part. I consider it a hack > rather than a systematic solution. I can easily imagine that we just > find out other call sites that would cause over reclaim or similar If a __GFP_NORETRY allocation does overreclaim - it could be fixed by returning NULL instead of doing overreclaim. The specification says that callers must handle failure of __GFP_NORETRY allocations. So yes - if you think that just skipping throttling on __GFP_NORETRY could cause excessive CPU consumption trying to reclaim unreclaimable pages or something like that - then you can add more points where the __GFP_NORETRY is failed with NULL to avoid the excessive CPU consumption. > problems because they are not throttled on too many dirty pages due to > congested storage. What are we going to then? Add another magic gfp > flag? Really, try to not add even more subtle side effects for gfp > flags. We _do_ have ways to accomplish what your particular usecase > requires. > > -- > Michal Hocko > SUSE Labs Mikulas
