| Message ID | 20200219210103.32400-12-willy@infradead.org (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | Change readahead API | expand |
On 2/19/20 1:00 PM, 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> > --- > mm/readahead.c | 15 +++++++++------ > 1 file changed, 9 insertions(+), 6 deletions(-) > > diff --git a/mm/readahead.c b/mm/readahead.c > index 07cdfbf00f4b..ace611f4bf05 100644 > --- a/mm/readahead.c > +++ b/mm/readahead.c > @@ -166,8 +166,6 @@ 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); > @@ -179,22 +177,27 @@ void __do_page_cache_readahead(struct address_space *mapping, > ._nr_pages = 0, > }; > 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; > + if (index + nr_to_read < index) > + nr_to_read = ULONG_MAX - index + 1; > + if (index + nr_to_read >= end_index) > + nr_to_read = end_index - index + 1; This tiny patch made me pause, because I wasn't sure at first of the exact intent of the lines above. Once I worked it out, it seemed like it might be helpful (or overkill??) to add a few hints for the reader, especially since there are no hints in the function's (minimal) documentation header. What do you think of this? /* * If we can't read *any* pages without going past the inodes's isize * limit, give up entirely: */ if (index > end_index) return; /* Cap nr_to_read, in order to avoid overflowing the ULONG type: */ if (index + nr_to_read < index) nr_to_read = ULONG_MAX - index + 1; /* Cap nr_to_read, to avoid reading past the inode's isize limit: */ if (index + nr_to_read >= end_index) nr_to_read = end_index - index + 1; Either way, it looks corrected written to me, so: Reviewed-by: John Hubbard <jhubbard@nvidia.com> thanks,
On Thu, Feb 20, 2020 at 07:50:39PM -0800, John Hubbard wrote: > This tiny patch made me pause, because I wasn't sure at first of the exact > intent of the lines above. Once I worked it out, it seemed like it might > be helpful (or overkill??) to add a few hints for the reader, especially since > there are no hints in the function's (minimal) documentation header. What > do you think of this? > > /* > * If we can't read *any* pages without going past the inodes's isize > * limit, give up entirely: > */ > if (index > end_index) > return; > > /* Cap nr_to_read, in order to avoid overflowing the ULONG type: */ > if (index + nr_to_read < index) > nr_to_read = ULONG_MAX - index + 1; > > /* Cap nr_to_read, to avoid reading past the inode's isize limit: */ > if (index + nr_to_read >= end_index) > nr_to_read = end_index - index + 1; A little verbose for my taste ... How about this? 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;
On 2/21/20 7:35 AM, Matthew Wilcox wrote: > On Thu, Feb 20, 2020 at 07:50:39PM -0800, John Hubbard wrote: >> This tiny patch made me pause, because I wasn't sure at first of the exact >> intent of the lines above. Once I worked it out, it seemed like it might >> be helpful (or overkill??) to add a few hints for the reader, especially since >> there are no hints in the function's (minimal) documentation header. What >> do you think of this? >> >> /* >> * If we can't read *any* pages without going past the inodes's isize >> * limit, give up entirely: >> */ >> if (index > end_index) >> return; >> >> /* Cap nr_to_read, in order to avoid overflowing the ULONG type: */ >> if (index + nr_to_read < index) >> nr_to_read = ULONG_MAX - index + 1; >> >> /* Cap nr_to_read, to avoid reading past the inode's isize limit: */ >> if (index + nr_to_read >= end_index) >> nr_to_read = end_index - index + 1; > > A little verbose for my taste ... How about this? Mine too, actually. :) I think your version below looks good. thanks,
diff --git a/mm/readahead.c b/mm/readahead.c index 07cdfbf00f4b..ace611f4bf05 100644 --- a/mm/readahead.c +++ b/mm/readahead.c @@ -166,8 +166,6 @@ 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); @@ -179,22 +177,27 @@ void __do_page_cache_readahead(struct address_space *mapping, ._nr_pages = 0, }; 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; + if (index + nr_to_read < index) + nr_to_read = ULONG_MAX - index + 1; + if (index + nr_to_read >= end_index) + nr_to_read = end_index - index + 1; /* * Preallocate as many pages as we will need. */ for (i = 0; i < nr_to_read; i++) { - if (index + i > end_index) - break; + struct page *page = xa_load(&mapping->i_pages, index + i); BUG_ON(index + i != rac._index + rac._nr_pages); - page = xa_load(&mapping->i_pages, index + i); if (page && !xa_is_value(page)) { /* * Page already present? Kick off the current batch of