Re: [f2fs-dev] [PATCH V3 6/6] xfs: ignore discard return value

Yongpeng Yang
2025-11-26T09:48:35Z

On 11/26/25 17:14, Yongpeng Yang wrote:
> On 11/26/25 16:07, Chaitanya Kulkarni via Linux-f2fs-devel wrote:
>> On 11/25/25 18:37, Yongpeng Yang wrote:
>>>> diff --git a/fs/xfs/xfs_discard.c b/fs/xfs/xfs_discard.c
>>>> index 6917de832191..b6ffe4807a11 100644
>>>> --- a/fs/xfs/xfs_discard.c
>>>> +++ b/fs/xfs/xfs_discard.c
>>>> @@ -108,7 +108,7 @@ xfs_discard_endio(
>>>>     * list. We plug and chain the bios so that we only need a single
>>>> completion
>>>>     * call to clear all the busy extents once the discards are
>>>> complete.
>>>>     */
>>>> -int
>>>> +void
>>>>    xfs_discard_extents(
>>>>        struct xfs_mount    *mp,
>>>>        struct xfs_busy_extents    *extents)
>>>> @@ -116,7 +116,6 @@ xfs_discard_extents(
>>>>        struct xfs_extent_busy    *busyp;
>>>>        struct bio        *bio = NULL;
>>>>        struct blk_plug        plug;
>>>> -    int            error = 0;
>>>>          blk_start_plug(&plug);
>>>>        list_for_each_entry(busyp, &extents->extent_list, list) {
>>>> @@ -126,18 +125,10 @@ xfs_discard_extents(
>>>>              trace_xfs_discard_extent(xg, busyp->bno, busyp->length);
>>>>    -        error = __blkdev_issue_discard(btp->bt_bdev,
>>>> +        __blkdev_issue_discard(btp->bt_bdev,
>>>>                    xfs_gbno_to_daddr(xg, busyp->bno),
>>>>                    XFS_FSB_TO_BB(mp, busyp->length),
>>>>                    GFP_KERNEL, &bio);
>>>
>>> If blk_alloc_discard_bio() fails to allocate a bio inside
>>> __blkdev_issue_discard(), this may lead to an invalid loop in
>>> list_for_each_entry{}. Instead of using __blkdev_issue_discard(), how
>>> about allocate and submit the discard bios explicitly in
>>> list_for_each_entry{}?
>>>
>>> Yongpeng,
>>
>>
>> Calling __blkdev_issue_discard() keeps managing all the bio with the
>> appropriate GFP mask, so the semantics stay the same. You are just
>> moving memory allocation to the caller and potentially looking at
>> implementing retry on bio allocation failure.
>>
>> The retry for discard bio memory allocation is not desired I think,
>> since it's only a hint to the controller.
>
> Agreed. I'm not trying to retry bio allocation inside the
> list_for_each_entry{} loop. Instead, since blk_alloc_discard_bio()
> returning NULL cannot reliably indicate whether the failure is due to
> bio allocation failure, it could also be caused by 'bio_sects == 0', I'd
> like to allocate the bio explicitly.
>
>>
>> This patch is simply cleaning up dead error-handling branches at the
>> callers no behavioral changes intended.
>>
>> What maybe useful is to stop iterating once we fail to allocate the
>> bio [1].
>>
>> -ck
>>
>> [1] Potential addition on the top of this to exit early in discard loop
>>       on bio allocation failure.
>>
>> diff --git a/fs/xfs/xfs_discard.c b/fs/xfs/xfs_discard.c
>> index b6ffe4807a11..1519f708bb79 100644
>> --- a/fs/xfs/xfs_discard.c
>> +++ b/fs/xfs/xfs_discard.c
>> @@ -129,6 +129,13 @@ xfs_discard_extents(
>>                                   xfs_gbno_to_daddr(xg, busyp->bno),
>>                                   XFS_FSB_TO_BB(mp, busyp->length),
>>                                   GFP_KERNEL, &bio);
>> +               /*
>> +                * We failed to allocate bio instead of continuing the
>> loop
>> +                * so it will lead to inconsistent discards to the disk
>> +                * exit early and jump into xfs_discard_busy_clear().
>> +                */
>> +               if (!bio)
>> +                       break;
>
> I noticed that as long as XFS_FSB_TO_BB(mp, busyp->length) is greater
> than 0 and there is no bio allocation failure, __blkdev_issue_discard()
> will never return NULL. I'm not familiar with this part of the xfs, so
> I'm not sure whether there are cases where 'XFS_FSB_TO_BB(mp,
> busyp->length)' could be 0. If such cases do not exist, then
> checking whether the bio is NULL should be sufficient.
>
> Yongpeng,

If __blkdev_issue_discard() requires multiple calls to
blk_alloc_discard_bio(), once the first bio allocation succeeds, it will
never result in bio == NULL, meaning that any subsequent bio allocation
failures cannot be detected.

Yongpeng,

>
>>           }
>>           if (bio) {
>> > If we keep looping after the first bio == NULL, the rest of the
>> range is
>> guaranteed to be inconsistent anyways, because every subsequent iteration
>> will fall into one of three cases:
>>
>> - The allocator keeps returning NULL, so none of the remaining LBAs
>> receive
>>     discard.
>> - Rest of the allocator succeeds, but we’ve already skipped a chunk,
>> leaving
>>     a hole in the discard range.
>> - We get intermittent successes, which produces alternating chunks of
>>     discarded and undiscarded blocks.
>>
>> In each of those scenarios, the disk ends up with a partially discarded
>> range, so the correct fix is to break out of the loop immediately and
>> proceed to xfs_discard_busy_clear() once the very first allocation fails.