On 8/30/24 11:14, Yafang Shao wrote: > On Thu, Aug 29, 2024 at 10:29 PM Dave Chinner <david@xxxxxxxxxxxxx> wrote: > > Hello Dave, > > I've noticed that XFS has increasingly replaced kmem_alloc() with > __GFP_NOFAIL. For example, in kernel 4.19.y, there are 0 instances of > __GFP_NOFAIL under fs/xfs, but in kernel 6.1.y, there are 41 > occurrences. In kmem_alloc(), there's an explicit > memalloc_retry_wait() to throttle the allocator under heavy memory > pressure, which aligns with your filesystem design. However, using > __GFP_NOFAIL removes this throttling mechanism, potentially causing > issues when the system is under heavy memory load. I'm concerned that > this shift might not be a beneficial trend. > > We have been using XFS for our big data servers for years, and it has > consistently performed well with older kernels like 4.19.y. However, > after upgrading all our servers from 4.19.y to 6.1.y over the past two > years, we have frequently encountered livelock issues caused by memory > exhaustion. To mitigate this, we've had to limit the RSS of > applications, which isn't an ideal solution and represents a worrying > trend. By "livelock issues caused by memory exhaustion" you mean the long-standing infamous issue that the system might become thrashing for the remaining small amount of page cache, and anonymous memory being swapped out/in, instead of issuing OOM, because there's always just enough progress of the reclaim to keep going, but the system isn't basically doing anything else? I think that's related to near-exhausted memory by userspace, so I'm not sure why XFS would be to blame here. That said, if memalloc_retry_wait() is indeed a useful mechanism, maybe we could perform it inside the page allocator itself for __GFP_NOFAIL?
