Messages in this thread | | | Date | Tue, 28 Jan 2014 12:03:01 -0800 | From | Andrew Morton <> | Subject | Re: [PATCH] mm: readahead: fix do_readahead for no readpage(s) |
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On Tue, 28 Jan 2014 11:14:19 +0000 Mark Rutland <mark.rutland@arm.com> wrote:
> Commit 63d0f0a3c7e1 (mm/readahead.c:do_readhead(): don't check for > ->readpage) unintentionally made do_readahead return 0 for all valid > files regardless of whether readahead was supported, rather than the > expected -EINVAL. This gets forwarded on to userspace, and results in > sys_readahead appearing to succeed in cases that don't make sense (e.g. > when called on pipes or sockets). This issue is detected by the LTP > readahead01 testcase.
How can this be?
: static ssize_t : do_readahead(struct address_space *mapping, struct file *filp, : pgoff_t index, unsigned long nr) : { : if (!mapping || !mapping->a_ops) : return -EINVAL; : : return force_page_cache_readahead(mapping, filp, index, nr); : }
and
: int force_page_cache_readahead(struct address_space *mapping, struct file *filp, : pgoff_t offset, unsigned long nr_to_read) : { : if (unlikely(!mapping->a_ops->readpage && !mapping->a_ops->readpages)) : return -EINVAL;
Clearly, do_readahead() will return -EINVAL if neither ->readpage or ->readpages are implemented.
I can see that the behaviour would change if the address_space implements only one of ->readpage and ->readpages, but that doesn't appear to match your description and the new behaviour is correct - we can now perform readahead for address_spaces which implement ->readpages and not ->readpage (which would be odd and might not work for other reasons..).
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