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Re: [PATCH] Fix up xfs_buf_associate_memory()

To: Lachlan McIlroy <lachlan@xxxxxxx>
Subject: Re: [PATCH] Fix up xfs_buf_associate_memory()
From: Christoph Hellwig <hch@xxxxxxxxxxxxx>
Date: Fri, 23 Nov 2007 13:43:02 +0000
Cc: xfs-dev <xfs-dev@xxxxxxx>, xfs-oss <xfs@xxxxxxxxxxx>
In-reply-to: <47465712.1050000@xxxxxxx>
References: <47465712.1050000@xxxxxxx>
Sender: xfs-bounce@xxxxxxxxxxx
User-agent: Mutt/1.4.2.3i
On Fri, Nov 23, 2007 at 03:29:06PM +1100, Lachlan McIlroy wrote:
> Fixed a few bugs in xfs_buf_associate_memory() including:
> 
> - calculation of 'page_count' was incorrect as it did not
>   consider the offset of 'mem' into the first page.  The
>   logic to bump 'page_count' didn't work if 'len' was <=
>   PAGE_CACHE_SIZE (ie offset = 3k, len = 2k).
> - setting b_buffer_length to 'len' is incorrect if
>   'offset' is > 0.  Set it to the total length of the
>   buffer.
> - I suspect that passing a non-aligned address into
>   mem_to_page() for the first page may have been causing
>   issues - don't know but just tidy up that code anyway.
> 
> These fixes prevent an data corruption issue that can
> occur during log replay.

Last time I tried to clean up this gem everything went bezerk, so be
aware :)


--- fs/xfs/linux-2.6/xfs_buf.c_1.247    2007-11-23 12:03:16.000000000 +1100
+++ fs/xfs/linux-2.6/xfs_buf.c  2007-11-23 12:02:32.000000000 +1100
@@ -726,14 +726,14 @@ xfs_buf_associate_memory(
        int                     rval;
        int                     i = 0;
        size_t                  ptr;
+       size_t                  buflen;
        off_t                   offset;
        int                     page_count;
 
+       ptr = (size_t) mem & PAGE_CACHE_MASK;
+       offset = (off_t) mem - (off_t) ptr;

Casting pointers to size_t or off_t makes little sense, these should be
unsigned long.  And using a variable name of ptr is quite odd :)

+       while (i < bp->b_page_count) {
+               bp->b_pages[i++] = mem_to_page((void *)ptr);
                ptr += PAGE_CACHE_SIZE;
        }

This could be much cleaner written as:

        for (i = 0; i < bp->b_page_count; i++) {
                bp->b_pages[i] = mem_to_page((void *)ptr);
                ptr += PAGE_CACHE_SIZE;
        }


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