From mboxrd@z Thu Jan 1 00:00:00 1970 From: Eric Biggers Date: Fri, 20 Mar 2020 09:58:28 -0700 Subject: [Cluster-devel] [PATCH v9 12/25] mm: Move end_index check out of readahead loop In-Reply-To: <20200320142231.2402-13-willy@infradead.org> References: <20200320142231.2402-1-willy@infradead.org> <20200320142231.2402-13-willy@infradead.org> Message-ID: <20200320165828.GB851@sol.localdomain> List-Id: To: cluster-devel.redhat.com MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit On Fri, Mar 20, 2020 at 07:22:18AM -0700, Matthew Wilcox wrote: > From: "Matthew Wilcox (Oracle)" > > By reducing nr_to_read, we can eliminate this check from inside the loop. > > Signed-off-by: Matthew Wilcox (Oracle) > Reviewed-by: John Hubbard > Reviewed-by: William Kucharski > --- > mm/readahead.c | 17 +++++++++++------ > 1 file changed, 11 insertions(+), 6 deletions(-) > > diff --git a/mm/readahead.c b/mm/readahead.c > index d01531ef9f3c..a37b68f66233 100644 > --- a/mm/readahead.c > +++ b/mm/readahead.c > @@ -167,8 +167,6 @@ void __do_page_cache_readahead(struct address_space *mapping, > unsigned long lookahead_size) > { > struct inode *inode = mapping->host; > - struct page *page; > - unsigned long end_index; /* The last page we want to read */ > LIST_HEAD(page_pool); > loff_t isize = i_size_read(inode); > gfp_t gfp_mask = readahead_gfp_mask(mapping); > @@ -178,22 +176,29 @@ void __do_page_cache_readahead(struct address_space *mapping, > ._index = index, > }; > unsigned long i; > + pgoff_t end_index; /* The last page we want to read */ > > if (isize == 0) > return; > > - end_index = ((isize - 1) >> PAGE_SHIFT); > + end_index = (isize - 1) >> PAGE_SHIFT; > + if (index > end_index) > + return; > + /* Avoid wrapping to the beginning of the file */ > + if (index + nr_to_read < index) > + nr_to_read = ULONG_MAX - index + 1; > + /* Don't read past the page containing the last byte of the file */ > + if (index + nr_to_read >= end_index) > + nr_to_read = end_index - index + 1; There seem to be a couple off-by-one errors here. Shouldn't it be: /* Avoid wrapping to the beginning of the file */ if (index + nr_to_read < index) nr_to_read = ULONG_MAX - index; /* Don't read past the page containing the last byte of the file */ if (index + nr_to_read > end_index) nr_to_read = end_index - index + 1; I.e., 'ULONG_MAX - index' rather than 'ULONG_MAX - index + 1', so that 'index + nr_to_read' is then ULONG_MAX rather than overflowed to 0. Then 'index + nr_to_read > end_index' rather 'index + nr_to_read >= end_index', since otherwise nr_to_read can be increased by 1 rather than decreased or stay the same as expected. - Eric