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? -- Michal Hocko SUSE Labs
