On Thu, 28 Nov 2013, Michal Hocko wrote: > > Ok, so let's forget about GFP_KERNEL | __GFP_NOFAIL since anything doing > > __GFP_FS should not be holding such locks, we have some of those in the > > drivers code and that makes sense that they are doing GFP_KERNEL. > > > > Focusing on the GFP_NOFS | __GFP_NOFAIL allocations in the filesystem > > code, the kernel oom killer independent of memcg never gets called because > > !__GFP_FS and they'll simply loop around the page allocator forever. > > > > In the past, Andrew has expressed the desire to get rid of __GFP_NOFAIL > > entirely since it's flawed when combined with GFP_NOFS (and GFP_KERNEL | > > __GFP_NOFAIL could simply be reimplemented in the caller) because of the > > reason you point out in addition to making it very difficult in the page > > allocator to free memory independent of memcg. > > > > So I'm wondering if we should just disable the oom killer in memcg for > > __GFP_NOFAIL as you've done here, but not bypass to the root memcg and > > just allow them to spin? I think we should be focused on the fixing the > > callers rather than breaking memcg isolation. > > What if the callers simply cannot deal with the allocation failure? > 84235de394d97 (fs: buffer: move allocation failure loop into the > allocator) describes one such case when __getblk_slow tries desperately > to grow buffers relying on the reclaim to free something. As there might > be no reclaim going on we are screwed. >
My suggestion is to spin, not return NULL. Bypassing to the root memcg can lead to a system oom condition whereas if memcg weren't involved at all the page allocator would just spin (because of !__GFP_FS). > That being said, while I do agree with you that we should strive for > isolation as much as possible there are certain cases when this is > impossible to achieve without seeing much worse consequences. For now, > we hope that __GFP_NOFAIL is used very scarcely. If that's true, why not bypass the per-zone min watermarks in the page allocator as well to allow these allocations to succeed? -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
