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[73.219.103.14]) by smtp.gmail.com with ESMTPSA id g9sm1108439qti.86.2020.11.04.13.30.07 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 04 Nov 2020 13:30:07 -0800 (PST) Date: Wed, 4 Nov 2020 16:30:05 -0500 From: Kent Overstreet To: "Matthew Wilcox (Oracle)" Cc: linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, hch@lst.de Subject: Re: [PATCH v2 02/18] mm/filemap: Remove dynamically allocated array from filemap_read Message-ID: <20201104213005.GB3365678@moria.home.lan> References: <20201104204219.23810-1-willy@infradead.org> <20201104204219.23810-3-willy@infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20201104204219.23810-3-willy@infradead.org> Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On Wed, Nov 04, 2020 at 08:42:03PM +0000, Matthew Wilcox (Oracle) wrote: > Increasing the batch size runs into diminishing returns. It's probably > better to make, eg, three calls to filemap_get_pages() than it is to > call into kmalloc(). I have to disagree. Working with PAGEVEC_SIZE pages is eventually going to be like working with 4k pages today, and have you actually read the slub code for the kmalloc fast path? It's _really_ fast, there's no atomic operations and it doesn't even have to disable preemption - which is why you never see it showing up in profiles ever since we switched to slub. It would however be better to have a standard abstraction for this rather than open coding it - perhaps adding it to the pagevec code. Please don't just drop it, though. > Signed-off-by: Matthew Wilcox (Oracle) > --- > mm/filemap.c | 15 ++------------- > 1 file changed, 2 insertions(+), 13 deletions(-) > > diff --git a/mm/filemap.c b/mm/filemap.c > index 23e3781b3aef..bb1c42d0223c 100644 > --- a/mm/filemap.c > +++ b/mm/filemap.c > @@ -2429,8 +2429,8 @@ ssize_t generic_file_buffered_read(struct kiocb *iocb, > struct file_ra_state *ra = &filp->f_ra; > struct address_space *mapping = filp->f_mapping; > struct inode *inode = mapping->host; > - struct page *pages_onstack[PAGEVEC_SIZE], **pages = NULL; > - unsigned int nr_pages = min_t(unsigned int, 512, > + struct page *pages[PAGEVEC_SIZE]; > + unsigned int nr_pages = min_t(unsigned int, PAGEVEC_SIZE, > ((iocb->ki_pos + iter->count + PAGE_SIZE - 1) >> PAGE_SHIFT) - > (iocb->ki_pos >> PAGE_SHIFT)); > int i, pg_nr, error = 0; > @@ -2441,14 +2441,6 @@ ssize_t generic_file_buffered_read(struct kiocb *iocb, > return 0; > iov_iter_truncate(iter, inode->i_sb->s_maxbytes); > > - if (nr_pages > ARRAY_SIZE(pages_onstack)) > - pages = kmalloc_array(nr_pages, sizeof(void *), GFP_KERNEL); > - > - if (!pages) { > - pages = pages_onstack; > - nr_pages = min_t(unsigned int, nr_pages, ARRAY_SIZE(pages_onstack)); > - } > - > do { > cond_resched(); > > @@ -2533,9 +2525,6 @@ ssize_t generic_file_buffered_read(struct kiocb *iocb, > > file_accessed(filp); > > - if (pages != pages_onstack) > - kfree(pages); > - > return written ? written : error; > } > EXPORT_SYMBOL_GPL(generic_file_buffered_read); > -- > 2.28.0 >