linux-kernel - Re: [RFC 0/4] convert create_page_buffers to create_folio

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Message-ID: <ZDoZCJHQXhVE2KZu@bombadil.infradead.org>
Date:   Fri, 14 Apr 2023 20:24:56 -0700
From:   Luis Chamberlain <mcgrof@...nel.org>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Hannes Reinecke <hare@...e.de>,
        Pankaj Raghav <p.raghav@...sung.com>, brauner@...nel.org,
        viro@...iv.linux.org.uk, akpm@...ux-foundation.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        gost.dev@...sung.com
Subject: Re: [RFC 0/4] convert create_page_buffers to create_folio_buffers

On Sat, Apr 15, 2023 at 03:31:54AM +0100, Matthew Wilcox wrote:
> On Fri, Apr 14, 2023 at 06:01:16PM -0700, Luis Chamberlain wrote:
> > a) dynamically allocate those now
> > b) do a cursory review of the users of that and prepare them
> >    to grok buffer heads which are blocksize based rather than
> >    PAGE_SIZE based. So we just try to kill MAX_BUF_PER_PAGE.
> > 
> > Without a) I think buffers after PAGE_SIZE won't get submit_bh() or lock for
> > bs > PAGE_SIZE right now.
> 
> Worse, we'll overflow the array and corrupt the stack.
> 
> This one is a simple fix ...
> 
> +++ b/fs/buffer.c
> @@ -2282,7 +2282,7 @@ int block_read_full_folio(struct folio *folio, get_block_t *get_block)
>  {
>         struct inode *inode = folio->mapping->host;
>         sector_t iblock, lblock;
> -       struct buffer_head *bh, *head, *arr[MAX_BUF_PER_PAGE];
> +       struct buffer_head *bh, *head;
>         unsigned int blocksize, bbits;
>         int nr, i;
>         int fully_mapped = 1;
> @@ -2335,7 +2335,6 @@ int block_read_full_folio(struct folio *folio, get_block_t *get_block)
>                         if (buffer_uptodate(bh))
>                                 continue;
>                 }
> -               arr[nr++] = bh;
>         } while (i++, iblock++, (bh = bh->b_this_page) != head);
>  
>         if (fully_mapped)
> @@ -2353,24 +2352,27 @@ int block_read_full_folio(struct folio *folio, get_block_t *get_block)
>         }
>  
>         /* Stage two: lock the buffers */
> -       for (i = 0; i < nr; i++) {
> -               bh = arr[i];
> +       bh = head;
> +       do {
>                 lock_buffer(bh);
>                 mark_buffer_async_read(bh);
> -       }
> +               bh = bh->b_this_page;
> +       } while (bh != head);
>  
>         /*
>          * Stage 3: start the IO.  Check for uptodateness
>          * inside the buffer lock in case another process reading
>          * the underlying blockdev brought it uptodate (the sct fix).
>          */
> -       for (i = 0; i < nr; i++) {
> -               bh = arr[i];
> +       bh = head;
> +       do {
>                 if (buffer_uptodate(bh))
>                         end_buffer_async_read(bh, 1);
>                 else
>                         submit_bh(REQ_OP_READ, bh);
> -       }
> +               bh = bh->b_this_page;
> +       } while (bh != head);
> +
>         return 0;

I thought of that but I saw that the loop that assigns the arr only
pegs a bh if we don't "continue" for certain conditions, which made me
believe that we only wanted to keep on the array as non-null items which
meet the initial loop's criteria. If that is not accurate then yes,
the simplication is nice!

  Luis