linux-kernel - Re: [PATCH] bcachefs: Switch to memalloc_flags

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Message-ID: <f62e400e-49ab-4d0a-b2e2-c3bbb66c2ab1@suse.cz>
Date: Fri, 30 Aug 2024 17:25:37 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: Yafang Shao <laoar.shao@...il.com>, Dave Chinner <david@...morbit.com>
Cc: Kent Overstreet <kent.overstreet@...ux.dev>,
 Michal Hocko <mhocko@...e.com>, Matthew Wilcox <willy@...radead.org>,
 linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, Dave Chinner <dchinner@...hat.com>
Subject: Re: [PATCH] bcachefs: Switch to memalloc_flags_do() for vmalloc
 allocations

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