Re: [PATCH RFC ver.B] btrfs: scrub: Don't use inode pages for device replace

[Nikolay Borisov <nborisov@suse.com>]

On  5.06.2018 17:24, David Sterba wrote:
> On Tue, Jun 05, 2018 at 10:14:51PM +0800, Qu Wenruo wrote:
>>>> The previous version (a completely different direction though) is much
>>>> smaller.
>>>> https://patchwork.kernel.org/patch/10440541/
>>>>
>>>> However personally speaking, I still prefer this one, as it's much simpler.
>>>
>>> As this will go to older stable kernels, I'd rather split that to more
>>> patches where the first one is
>>>
>>> --- a/fs/btrfs/scrub.c
>>> +++ b/fs/btrfs/scrub.c
>>> @@ -2799,7 +2799,7 @@ static int scrub_extent(struct scrub_ctx *sctx, struct map_lookup *map,
>>>                         have_csum = scrub_find_csum(sctx, logical, csum);
>>>                         if (have_csum == 0)
>>>                                 ++sctx->stat.no_csum;
>>> -                       if (sctx->is_dev_replace && !have_csum) {
>>> +                       if (0 && sctx->is_dev_replace && !have_csum) {
>>>                                 ret = copy_nocow_pages(sctx, logical, l,
>>>                                                        mirror_num,
>>>                                                       physical_for_dev_replace);
>>> ---
>>>
>>> and then the whole callchain of copy_nocow_pages continues.
>>
>> Understood.
>> I could go this method.
>>
>> However I'm a little concerned about such "if (0 &&" usage.
>>
>> Although with gcc 8.1 it works without any warning, it still looks
>> pretty strange, as compiler could one day detect such dead branch and
>> find copy_nocow_pages() is never used.
> 
> I've checked that this does not produce any warnings, gcc 7.3.1. The
> condition looks strange but does what we want. The whole series will be
> in 4.18, the first patch in stable versions. If gcc does not warn today,
> it will not in the future in any of the versions.

WOuld it be possible to then take Qu's patch which deletes a lot of code
for 4.19?

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