On 2011-03-04, at 2:07 PM, Darrick J. Wong wrote:
> Ok, here's what I have so far. I took everyone's suggestions of where to add
> calls to wait_on_page_writeback, which seems to handle the multiple-write case
> adequately. Unfortunately, it is still possible to generate checksum errors by
> scribbling furiously on a mmap'd region, even after adding the writeback wait
> in the ext4 writepage function. Oddly, I couldn't break btrfs with mmap by
> removing its wait_for_page_writeback call, so I suspect there's a bit more
> going on in btrfs than I've been able to figure out.
Did you add this to ext4_page_mkwrite() or only ext4_writepage()? It wasn't clear from your description above.
> The set_memory_ro debugging trick didn't ferret out any write paths that I
> didn't catch... though it did have the effect of causing occasional fsync()
> deadlocks. I suppose I could sprinkle in a few more of those write calls to
> see what happens.
>
> Either way, I'm emailing to ask everyone's advice since I've run out of ideas.
> Or: Did I miss something?
>
> Thanks all for the feedback so far!
>
> --
> fs: Wait for page writeback when rewrite detected
>
> Signed-off-by: Darrick J. Wong <djwong@xxxxxxxxxx>
> ---
>
> fs/buffer.c | 4 +++-
> fs/ext4/inode.c | 3 +++
> mm/filemap.c | 15 +++++++++++++--
> 3 files changed, 19 insertions(+), 3 deletions(-)
>
> diff --git a/fs/buffer.c b/fs/buffer.c
> index 2219a76..39e934c 100644
> --- a/fs/buffer.c
> +++ b/fs/buffer.c
> @@ -2379,8 +2379,10 @@ block_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf,
> ret = VM_FAULT_OOM;
> else /* -ENOSPC, -EIO, etc */
> ret = VM_FAULT_SIGBUS;
> - } else
> + } else {
> + wait_on_page_writeback(page);
> ret = VM_FAULT_LOCKED;
> + }
>
> out:
> return ret;
> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> index 9f7f9e4..2364704 100644
> --- a/fs/ext4/inode.c
> +++ b/fs/ext4/inode.c
> @@ -2730,12 +2730,15 @@ static int ext4_writepage(struct page *page,
> struct inode *inode = page->mapping->host;
>
> trace_ext4_writepage(inode, page);
> +lock_page(page);
> size = i_size_read(inode);
> if (page->index == size >> PAGE_CACHE_SHIFT)
> len = size & ~PAGE_CACHE_MASK;
> else
> len = PAGE_CACHE_SIZE;
>
> +wait_on_page_writeback(page);
> +
> /*
> * If the page does not have buffers (for whatever reason),
> * try to create them using __block_write_begin. If this
> diff --git a/mm/filemap.c b/mm/filemap.c
> index 83a45d3..f201d80 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -2217,8 +2217,8 @@ EXPORT_SYMBOL(generic_file_direct_write);
> * Find or create a page at the given pagecache position. Return the locked
> * page. This function is specifically for buffered writes.
> */
> -struct page *grab_cache_page_write_begin(struct address_space *mapping,
> - pgoff_t index, unsigned flags)
> +struct page *__grab_cache_page_write_begin(struct address_space *mapping,
> + pgoff_t index, unsigned flags)
> {
> int status;
> struct page *page;
> @@ -2243,6 +2243,17 @@ repeat:
> }
> return page;
> }
> +struct page *grab_cache_page_write_begin(struct address_space *mapping,
> + pgoff_t index, unsigned flags)
> +{
> + struct page *p;
> +
> + p = __grab_cache_page_write_begin(mapping, index, flags);
> + if (p)
> + wait_on_page_writeback(p);
> +
> + return p;
> +}
> EXPORT_SYMBOL(grab_cache_page_write_begin);
>
> static ssize_t generic_perform_write(struct file *file,
> --
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Cheers, Andreas
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