linux-kernel - Re: [RFC PATCH 1/3] filemap: convert page_endio to folio

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Date:   Thu, 16 Mar 2023 08:24:04 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Pankaj Raghav <p.raghav@...sung.com>
Cc:     Christoph Hellwig <hch@...radead.org>, hubcap@...ibond.com,
        senozhatsky@...omium.org, martin@...ibond.com, willy@...radead.org,
        minchan@...nel.org, viro@...iv.linux.org.uk, brauner@...nel.org,
        axboe@...nel.dk, akpm@...ux-foundation.org,
        linux-kernel@...r.kernel.org, linux-block@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        gost.dev@...sung.com, mcgrof@...nel.org, devel@...ts.orangefs.org
Subject: Re: [RFC PATCH 1/3] filemap: convert page_endio to folio_endio

On Thu, Mar 16, 2023 at 11:04:54AM +0100, Pankaj Raghav wrote:
> It looks like this endio function is called when alloc_page is used (for
> partial IO) to trigger writeback from the user space `echo "idle" >
> /sys/block/zram0/writeback`.


Yes.

> I don't understand when you say the harm might not be horrible if we don't
> call folio_endio here. Do you think it is just safe to remove the call to
> folio_endio function?

I suspect so.  It doesn't seem like the involved pages are ever locked
or have the writeback set, so it should be fine.

> +               while ((folio = readahead_folio(rac))) {
> +                       folio_clear_uptodate(folio);
> +                       folio_set_error(folio);
> +                       folio_unlock(folio);
> +               }
> +               return;
> +       }
> +
> +       while ((folio = readahead_folio(rac))) {
> +               folio_mark_uptodate(folio);
> +               folio_unlock(folio);
>         }
>  }

Looks good.

> @@ -59,7 +59,8 @@ static void mpage_end_io(struct bio *bio)
> static struct bio *mpage_bio_submit(struct bio *bio)
>  {
> -       bio->bi_end_io = mpage_end_io;
> +       bio->bi_end_io = (op_is_write(bio_op(bio))) ? mpage_write_end_io :
> +                                                     mpage_read_end_io;
>         guard_bio_eod(bio);
>         submit_bio(bio);
>         return NULL;
> And mpage_{write,read}_end_io will iterate over the folio and call the
> respective functions.

Yes, although I'd do it with a good old if/else and with less braces.
It might make sense to split mpage_bio_submit as well, though.