On Sun, 4 Mar 2012, Hillf Danton wrote:
> In shmem_read_mapping_page_gfp() page is unlocked if no error returned,
> so the unlocked page has to valid.
>
> To guarantee that validity, when getting page, success result is feed
> back to caller only when page is valid.
>
> Signed-off-by: Hillf Danton <dhillf@xxxxxxxxx>
I don't understand your description, nor its relation to the patch.
NAK to the patch: when no page has previously been allocated, the
SGP_READ case avoids allocation and returns NULL - do_shmem_file_read
then copies the ZERO_PAGE instead, avoiding lots of unnecessary memory
allocation when reading a large sparse file.
Hugh
> ---
>
> --- a/mm/shmem.c Sun Mar 4 12:17:42 2012
> +++ b/mm/shmem.c Sun Mar 4 12:26:56 2012
> @@ -889,13 +889,13 @@ repeat:
> goto failed;
> }
>
> - if (page || (sgp == SGP_READ && !swap.val)) {
> + if (page) {
> /*
> * Once we can get the page lock, it must be uptodate:
> * if there were an error in reading back from swap,
> * the page would not be inserted into the filecache.
> */
> - BUG_ON(page && !PageUptodate(page));
> + BUG_ON(!PageUptodate(page));
> *pagep = page;
> return 0;
> }
> --
>
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