Re: [PATCH] bcachefs: Switch to memalloc_flags_do() for vmalloc allocations

[Vlastimil Babka <vbabka@suse.cz>]

On 8/30/24 11:14, Yafang Shao wrote:
> On Thu, Aug 29, 2024 at 10:29 PM Dave Chinner <david@fromorbit.com> 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?