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Wed, 24 Jun 2020 08:35:20 -0700 (PDT) Subject: Re: [PATCH 05/15] mm: allow read-ahead with IOCB_NOWAIT set From: Jens Axboe To: Matthew Wilcox , Dave Chinner Cc: io-uring@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, akpm@linux-foundation.org, Johannes Weiner References: <20200618144355.17324-1-axboe@kernel.dk> <20200618144355.17324-6-axboe@kernel.dk> <20200624010253.GB5369@dread.disaster.area> <20200624014645.GJ21350@casper.infradead.org> Message-ID: <70b0427c-7303-8f45-48bd-caa0562a2951@kernel.dk> Date: Wed, 24 Jun 2020 09:35:19 -0600 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.8.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: 81C191801ACA4 X-Spamd-Result: default: False [0.00 / 100.00] X-Rspamd-Server: rspam02 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 6/24/20 9:00 AM, Jens Axboe wrote: > On 6/23/20 7:46 PM, Matthew Wilcox wrote: >> On Wed, Jun 24, 2020 at 11:02:53AM +1000, Dave Chinner wrote: >>> On Thu, Jun 18, 2020 at 08:43:45AM -0600, Jens Axboe wrote: >>>> The read-ahead shouldn't block, so allow it to be done even if >>>> IOCB_NOWAIT is set in the kiocb. >>> >>> Doesn't think break preadv2(RWF_NOWAIT) semantics for on buffered >>> reads? i.e. this can now block on memory allocation for the page >>> cache, which is something RWF_NOWAIT IO should not do.... >> >> Yes. This eventually ends up in page_cache_readahead_unbounded() >> which gets its gfp flags from readahead_gfp_mask(mapping). >> >> I'd be quite happy to add a gfp_t to struct readahead_control. >> The other thing I've been looking into for other reasons is adding >> a memalloc_nowait_{save,restore}, which would avoid passing down >> the gfp_t. > > That was my first thought, having the memalloc_foo_save/restore for > this. I don't think adding a gfp_t to readahead_control is going > to be super useful, seems like the kind of thing that should be > non-blocking by default. We're already doing memalloc_nofs_save/restore in page_cache_readahead_unbounded(), so I think all we need is to just do a noio dance in generic_file_buffered_read() and that should be enough. diff --git a/mm/filemap.c b/mm/filemap.c index a5b1fa8f7ce4..c29d4b310ed6 100644 --- a/mm/filemap.c +++ b/mm/filemap.c @@ -41,6 +41,7 @@ #include #include #include +#include #include "internal.h" #define CREATE_TRACE_POINTS @@ -2011,6 +2012,7 @@ ssize_t generic_file_buffered_read(struct kiocb *iocb, struct address_space *mapping = filp->f_mapping; struct inode *inode = mapping->host; struct file_ra_state *ra = &filp->f_ra; + const bool nowait = (iocb->ki_flags & IOCB_NOWAIT) != 0; loff_t *ppos = &iocb->ki_pos; pgoff_t index; pgoff_t last_index; @@ -2044,9 +2046,15 @@ ssize_t generic_file_buffered_read(struct kiocb *iocb, page = find_get_page(mapping, index); if (!page) { + unsigned int flags; + + if (nowait) + flags = memalloc_noio_save(); page_cache_sync_readahead(mapping, ra, filp, index, last_index - index); + if (nowait) + memalloc_noio_restore(flags); page = find_get_page(mapping, index); if (unlikely(page == NULL)) goto no_cached_page; @@ -2070,7 +2078,7 @@ ssize_t generic_file_buffered_read(struct kiocb *iocb, error = wait_on_page_locked_async(page, iocb->ki_waitq); } else { - if (iocb->ki_flags & IOCB_NOWAIT) { + if (nowait) { put_page(page); goto would_block; } @@ -2185,7 +2193,7 @@ ssize_t generic_file_buffered_read(struct kiocb *iocb, } readpage: - if (iocb->ki_flags & IOCB_NOWAIT) { + if (nowait) { unlock_page(page); put_page(page); goto would_block; -- Jens Axboe