Re: [PATCH] block: skip redundant flush for O_DSYNC direct writes

On Thu, Aug 13, 2026 at 12:42 AM Christoph Hellwig <[email protected]> wrote:
>
> > +     bool dio_fua_done = false;
> >       ssize_t ret;
> >
> >       if (bdev_read_only(bdev))
> > @@ -763,9 +764,13 @@ static ssize_t blkdev_write_iter(struct kiocb *iocb, struct iov_iter *from)
> >
> >       if (iocb->ki_flags & IOCB_DIRECT) {
> >               ret = blkdev_direct_write(iocb, from);
> > -             if (ret >= 0 && iov_iter_count(from))
> > +             if (ret >= 0 && iov_iter_count(from)) {
> >                       ret = direct_write_fallback(iocb, from, ret,
> >                                       blkdev_buffered_write(iocb, from));
> > +             } else if (ret > 0 && iocb_is_dsync(iocb)) {
> > +                     /* FUA from dio_bio_write_op() already made it durable */
> > +                     dio_fua_done = true;
>
> This is kinda the wrong way aroud, it should just have a need_sync
> variable that is only set for the fallback.

Agreed, that reads much cleaner, I'll modify the flag in v2.

> But looking at this, the original logic here actually is very fishy.
> If we have a driver that does not actually support FUA, emulating it
> for every write command is a bad idea.  So this should be fixed to
> only do FUA when actually useful, and then return from the non-fallback
> implementations if that actually happened.

Let me make sure:
     blkdev_write_iter() then only skips generic_write_sync() when
     the direct-IO path actually issued REQ_FUA (i.e. iocb_is_dsync(iocb)
     && bdev_fua(bdev) && the write completed without falling back to
     buffered).  Is that the direction you had in mind?  If yes, I'll send patch
     v2 later.

Thanks for the review,
Zhenxian