Messages in this thread | | | Date | Fri, 20 Mar 2020 09:58:28 -0700 | From | Eric Biggers <> | Subject | Re: [PATCH v9 12/25] mm: Move end_index check out of readahead loop |
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On Fri, Mar 20, 2020 at 07:22:18AM -0700, Matthew Wilcox wrote: > From: "Matthew Wilcox (Oracle)" <willy@infradead.org> > > By reducing nr_to_read, we can eliminate this check from inside the loop. > > Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org> > Reviewed-by: John Hubbard <jhubbard@nvidia.com> > Reviewed-by: William Kucharski <william.kucharski@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
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