Matthew Wilcox
2020-Feb-17 18:45 UTC
[Ocfs2-devel] [PATCH v6 04/19] mm: Rearrange readahead loop
From: "Matthew Wilcox (Oracle)" <willy at infradead.org> Move the declaration of 'page' to inside the loop and move the 'kick off a fresh batch' code to the end of the function for easier use in subsequent patches. Signed-off-by: Matthew Wilcox (Oracle) <willy at infradead.org> --- mm/readahead.c | 21 +++++++++++++-------- 1 file changed, 13 insertions(+), 8 deletions(-) diff --git a/mm/readahead.c b/mm/readahead.c index 15329309231f..3eca59c43a45 100644 --- a/mm/readahead.c +++ b/mm/readahead.c @@ -154,7 +154,6 @@ 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); int page_idx; @@ -175,6 +174,7 @@ void __do_page_cache_readahead(struct address_space *mapping, * Preallocate as many pages as we will need. */ for (page_idx = 0; page_idx < nr_to_read; page_idx++) { + struct page *page; pgoff_t page_offset = offset + page_idx; if (page_offset > end_index) @@ -183,14 +183,14 @@ void __do_page_cache_readahead(struct address_space *mapping, page = xa_load(&mapping->i_pages, page_offset); if (page && !xa_is_value(page)) { /* - * Page already present? Kick off the current batch of - * contiguous pages before continuing with the next - * batch. + * Page already present? Kick off the current batch + * of contiguous pages before continuing with the + * next batch. This page may be the one we would + * have intended to mark as Readahead, but we don't + * have a stable reference to this page, and it's + * not worth getting one just for that. */ - if (readahead_count(&rac)) - read_pages(&rac, &page_pool, gfp_mask); - rac._nr_pages = 0; - continue; + goto read; } page = __page_cache_alloc(gfp_mask); @@ -201,6 +201,11 @@ void __do_page_cache_readahead(struct address_space *mapping, if (page_idx == nr_to_read - lookahead_size) SetPageReadahead(page); rac._nr_pages++; + continue; +read: + if (readahead_count(&rac)) + read_pages(&rac, &page_pool, gfp_mask); + rac._nr_pages = 0; } /* -- 2.25.0
Dave Chinner
2020-Feb-18 05:08 UTC
[Ocfs2-devel] [PATCH v6 04/19] mm: Rearrange readahead loop
On Mon, Feb 17, 2020 at 10:45:45AM -0800, Matthew Wilcox wrote:> From: "Matthew Wilcox (Oracle)" <willy at infradead.org> > > Move the declaration of 'page' to inside the loop and move the 'kick > off a fresh batch' code to the end of the function for easier use in > subsequent patches.Stale? the "kick off" code is moved to the tail of the loop, not the end of the function.> @@ -183,14 +183,14 @@ void __do_page_cache_readahead(struct address_space *mapping, > page = xa_load(&mapping->i_pages, page_offset); > if (page && !xa_is_value(page)) { > /* > - * Page already present? Kick off the current batch of > - * contiguous pages before continuing with the next > - * batch. > + * Page already present? Kick off the current batch > + * of contiguous pages before continuing with the > + * next batch. This page may be the one we would > + * have intended to mark as Readahead, but we don't > + * have a stable reference to this page, and it's > + * not worth getting one just for that. > */ > - if (readahead_count(&rac)) > - read_pages(&rac, &page_pool, gfp_mask); > - rac._nr_pages = 0; > - continue; > + goto read; > } > > page = __page_cache_alloc(gfp_mask); > @@ -201,6 +201,11 @@ void __do_page_cache_readahead(struct address_space *mapping, > if (page_idx == nr_to_read - lookahead_size) > SetPageReadahead(page); > rac._nr_pages++; > + continue; > +read: > + if (readahead_count(&rac)) > + read_pages(&rac, &page_pool, gfp_mask); > + rac._nr_pages = 0; > }Also, why? This adds a goto from branched code that continues, then adds a continue so the unbranched code doesn't execute the code the goto jumps to. In absence of any explanation, this isn't an improvement and doesn't make any sense... -- Dave Chinner david at fromorbit.com
John Hubbard
2020-Feb-18 22:33 UTC
[Ocfs2-devel] [PATCH v6 04/19] mm: Rearrange readahead loop
On 2/17/20 10:45 AM, Matthew Wilcox wrote:> From: "Matthew Wilcox (Oracle)" <willy at infradead.org> > > Move the declaration of 'page' to inside the loop and move the 'kick > off a fresh batch' code to the end of the function for easier use in > subsequent patches. > > Signed-off-by: Matthew Wilcox (Oracle) <willy at infradead.org> > --- > mm/readahead.c | 21 +++++++++++++-------- > 1 file changed, 13 insertions(+), 8 deletions(-) > > diff --git a/mm/readahead.c b/mm/readahead.c > index 15329309231f..3eca59c43a45 100644 > --- a/mm/readahead.c > +++ b/mm/readahead.c > @@ -154,7 +154,6 @@ 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); > int page_idx; > @@ -175,6 +174,7 @@ void __do_page_cache_readahead(struct address_space *mapping, > * Preallocate as many pages as we will need. > */ > for (page_idx = 0; page_idx < nr_to_read; page_idx++) { > + struct page *page; > pgoff_t page_offset = offset + page_idx; > > if (page_offset > end_index) > @@ -183,14 +183,14 @@ void __do_page_cache_readahead(struct address_space *mapping, > page = xa_load(&mapping->i_pages, page_offset); > if (page && !xa_is_value(page)) { > /* > - * Page already present? Kick off the current batch of > - * contiguous pages before continuing with the next > - * batch. > + * Page already present? Kick off the current batch > + * of contiguous pages before continuing with the > + * next batch. This page may be the one we would > + * have intended to mark as Readahead, but we don't > + * have a stable reference to this page, and it's > + * not worth getting one just for that. > */ > - if (readahead_count(&rac)) > - read_pages(&rac, &page_pool, gfp_mask); > - rac._nr_pages = 0; > - continue;A fine point: you'll get better readability and a less complex function by factoring that into a static subroutine, instead of jumping around with goto's. (This clearly wants to be a subroutine, and in fact you've effectively created one inside this function, at the "read:" label. Either way, though...> + goto read; > } > > page = __page_cache_alloc(gfp_mask); > @@ -201,6 +201,11 @@ void __do_page_cache_readahead(struct address_space *mapping, > if (page_idx == nr_to_read - lookahead_size) > SetPageReadahead(page); > rac._nr_pages++; > + continue; > +read: > + if (readahead_count(&rac)) > + read_pages(&rac, &page_pool, gfp_mask); > + rac._nr_pages = 0; > } > > /* >...no errors spotted, I'm confident that this patch is correct, Reviewed-by: John Hubbard <jhubbard at nvidia.com> thanks, -- John Hubbard NVIDIA