Re: [PATCH 5/6] XFS: remove congestion_wait() loop from kmem_alloc()

Dave Chinner
2021-09-14T01:31:26Z

On Tue, Sep 14, 2021 at 10:13:04AM +1000, NeilBrown wrote:
> Documentation commment in gfp.h discourages indefinite retry loops on
> ENOMEM and says of __GFP_NOFAIL that it
>
> is definitely preferable to use the flag rather than opencode
> endless loop around allocator.
>
> So remove the loop, instead specifying __GFP_NOFAIL if KM_MAYFAIL was
> not given.
>
> Signed-off-by: NeilBrown <neilb@suse.de>
> ---
> fs/xfs/kmem.c | 16 ++++------------
> 1 file changed, 4 insertions(+), 12 deletions(-)
>
> diff --git a/fs/xfs/kmem.c b/fs/xfs/kmem.c
> index 6f49bf39183c..f545f3633f88 100644
> --- a/fs/xfs/kmem.c
> +++ b/fs/xfs/kmem.c
> @@ -13,19 +13,11 @@ kmem_alloc(size_t size, xfs_km_flags_t flags)
> {
> int retries = 0;
> gfp_t lflags = kmem_flags_convert(flags);
> - void *ptr;
>
> trace_kmem_alloc(size, flags, _RET_IP_);
>
> - do {
> - ptr = kmalloc(size, lflags);
> - if (ptr || (flags & KM_MAYFAIL))
> - return ptr;
> - if (!(++retries % 100))
> - xfs_err(NULL,
> - "%s(%u) possible memory allocation deadlock size %u in %s (mode:0x%x)",
> - current->comm, current->pid,
> - (unsigned int)size, __func__, lflags);
> - congestion_wait(BLK_RW_ASYNC, HZ/50);
> - } while (1);
> + if (!(flags & KM_MAYFAIL))
> + lflags |= __GFP_NOFAIL;
> +
> + return kmalloc(size, lflags);
> }

Which means we no longer get warnings about memory allocation
failing - kmem_flags_convert() sets __GFP_NOWARN for all allocations
in this loop. Hence we'll now get silent deadlocks through this code
instead of getting warnings that memory allocation is failing
repeatedly.

I also wonder about changing the backoff behaviour here (it's a 20ms
wait right now because there are not early wakeups) will affect the
behaviour, as __GFP_NOFAIL won't wait for that extra time between
allocation attempts....

And, of course, how did you test this? Sometimes we see
unpredicted behaviours as a result of "simple" changes like this
under low memory conditions...

Cheers,

Dave.
--
Dave Chinner
david@fromorbit.com