On Thu, 7 Jun 2007 11:46:53 +1000 NeilBrown <[EMAIL PROTECTED]> wrote:
> do_generic_mapping_read currently samples the i_size at the start > and doesn't do so again unless it needs to call ->readpage to load > a page. After ->readpage it has to re-sample i_size as a truncate > may have caused that page to be filled with zeros, and the read() > call should not see these. > > However there are other activities that might cause ->readpage to be > called on a page between the time that do_generic_mapping_read > samples i_size and when it finds that it has an uptodate page. These > include at least read-ahead and possibly another thread performing a > read. > > So do_generic_mapping_read must sample i_size *after* it has an > uptodate page. Thus the current sampling at the start and after a read > can be replaced with a sampling before the copy-out. > > The same change applied to __generic_file_splice_read. > > Note that this fixes any race with truncate_complete_page, but does > not fix a possible race with truncate_partial_page. If a partial > truncate happens after do_generic_mapping_read samples i_size and > before the copy_out, the nuls that truncate_partial_page place in the > page could be copied out incorrectly. > > I think the best fix for that is to *not* zero out parts of the page > in truncate_partial_page, but rather to zero out the tail of a page > when increasing i_size. Is this really worth fixing? The code still has races there - for example, another process can come after we've got the page, truncate it off the file then write new data. So this thread ends up copying out-of-date page contents out to userspace. The application is being silly, and the silly application gets wrong data: either out-of-date or zeroed. afacit the patch shrinks the timing windows significantly, but at a cost: moving a bunch of 64-bit arithmetic and seqlock operations into the inner loop. We could plug all this in a more predictable fashion by locking the page, but then we have another instance of the read-a-page-into-a-mmapped-copy-of-itself deadlock (I think - maybe it can't happen because of page uptodateness checks). - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
