Re: [PATCH] clear PageError bit in msync & fsync

[Valerie Aurora <vaurora@redhat.com>]

On Tue, Nov 09, 2010 at 11:44:22AM -0500, Rik van Riel wrote:
> Temporary IO failures, eg. due to loss of both multipath paths, can
> permanently leave the PageError bit set on a page, resulting in
> msync or fsync returning -EIO over and over again, even if IO is
> now getting to the disk correctly.
> 
> We already clear the AS_ENOSPC and AS_IO bits in mapping->flags in
> the filemap_fdatawait_range function.  Also clearing the PageError
> bit on the page allows subsequent msync or fsync calls on this file
> to return without an error, if the subsequent IO succeeds.
> 
> Unfortunately data written out in the msync or fsync call that
> returned -EIO can still get lost, because the page dirty bit appears
> to not get restored on IO error.  However, the alternative could be
> potentially all of memory filling up with uncleanable dirty pages,
> hanging the system, so there is no nice choice here...
> 
> Signed-off-by: Rik van Riel <riel@redhat.com>
> 
> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> index 5f38c46..4805fde 100644
> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -198,7 +198,7 @@ static inline int __TestClearPage##uname(struct page *page) { return 0; }
>  struct page;	/* forward declaration */
>  
>  TESTPAGEFLAG(Locked, locked) TESTSETFLAG(Locked, locked)
> -PAGEFLAG(Error, error)
> +PAGEFLAG(Error, error) TESTCLEARFLAG(Error, error)
>  PAGEFLAG(Referenced, referenced) TESTCLEARFLAG(Referenced, referenced)
>  PAGEFLAG(Dirty, dirty) TESTSCFLAG(Dirty, dirty) __CLEARPAGEFLAG(Dirty, dirty)
>  PAGEFLAG(LRU, lru) __CLEARPAGEFLAG(LRU, lru)
> diff --git a/mm/filemap.c b/mm/filemap.c
> index 61ba5e4..ba27b83 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -296,7 +296,7 @@ int filemap_fdatawait_range(struct address_space *mapping, loff_t start_byte,
>  				continue;
>  
>  			wait_on_page_writeback(page);
> -			if (PageError(page))
> +			if (TestClearPageError(page))
>  				ret = -EIO;
>  		}
>  		pagevec_release(&pvec);
> 

I don't think losing the dirty bit is a problem.  If the msync/fsync
fails with EIO, it's userspace's job to reissue the write, not the
kernel's.

Returning EIO only once per actual error looks correct to me.

Acked-by: Valerie Aurora <vaurora@redhat.com>

-VAL