Eric Biggers
2020-Mar-20 16:58 UTC
[Ocfs2-devel] [PATCH v9 12/25] mm: Move end_index check out of readahead loop
On Fri, Mar 20, 2020 at 07:22:18AM -0700, Matthew Wilcox wrote:> From: "Matthew Wilcox (Oracle)" <willy at infradead.org> > > By reducing nr_to_read, we can eliminate this check from inside the loop. > > Signed-off-by: Matthew Wilcox (Oracle) <willy at infradead.org> > Reviewed-by: John Hubbard <jhubbard at nvidia.com> > Reviewed-by: William Kucharski <william.kucharski at oracle.com> > --- > 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
Matthew Wilcox
2020-Mar-20 17:30 UTC
[Ocfs2-devel] [PATCH v9 12/25] mm: Move end_index check out of readahead loop
On Fri, Mar 20, 2020 at 09:58:28AM -0700, Eric Biggers wrote:> On Fri, Mar 20, 2020 at 07:22:18AM -0700, Matthew Wilcox wrote: > > + /* 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;I think it's right. Imagine that index is ULONG_MAX. We should read one page (the one at ULONG_MAX). That would be ULONG_MAX - ULONG_MAX + 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; > > 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.Ooh, I missed the overflow case here. It should be: + if (index + nr_to_read - 1 > end_index) + nr_to_read = end_index - index + 1; Let's say index comes in at ULONG_MAX - 2, end_index is ULONG_MAX - 1 and nr_to_read is 8. The first condition triggers and nr_to_read is reduced to 3. But then the second condition wouldn't trigger because ULONG_MAX - 2 + 3 is 0. With the rewrite I have in this message, ULONG_MAX - 2 + 3 - 1 is ULONG_MAX, which is > ULONG_MAX - 1. So the condition triggers and nr_to_read becomes (ULONG_MAX - 1) - (ULONG_MAX - 2) + 1. Which is -1 + 2 + 1, which is 2. Which is the right answer because we want to read two pages; the one at ULONG_MAX - 2 and the one at ULONG_MAX - 1. Thank you!