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

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Date:   Sat, 15 Apr 2023 03:31:54 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Luis Chamberlain <mcgrof@...nel.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 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;
 }
 EXPORT_SYMBOL(block_read_full_folio);