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[24.61.119.116]) by smtp.gmail.com with ESMTPSA id f25-20020ac84659000000b0035ccd148026sm1791612qto.69.2022.09.21.08.39.17 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 21 Sep 2022 08:39:18 -0700 (PDT) Date: Wed, 21 Sep 2022 11:39:15 -0400 From: Brian Foster To: Sarthak Kukreti Cc: dm-devel@redhat.com, linux-block@vger.kernel.org, linux-ext4@vger.kernel.org, linux-kernel@vger.kernel.org, virtualization@lists.linux-foundation.org, Jens Axboe , "Michael S . Tsirkin" , Jason Wang , Paolo Bonzini , Stefan Hajnoczi , Alasdair Kergon , Mike Snitzer , Theodore Ts'o , Andreas Dilger , Bart Van Assche , Daniil Lunev , Evan Green , Gwendal Grignou Subject: Re: [PATCH RFC 4/8] fs: Introduce FALLOC_FL_PROVISION Message-ID: References: <20220915164826.1396245-1-sarthakkukreti@google.com> <20220915164826.1396245-5-sarthakkukreti@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Sep 16, 2022 at 02:02:31PM -0700, Sarthak Kukreti wrote: > On Fri, Sep 16, 2022 at 4:56 AM Brian Foster wrote: > > > > On Thu, Sep 15, 2022 at 09:48:22AM -0700, Sarthak Kukreti wrote: > > > From: Sarthak Kukreti > > > > > > FALLOC_FL_PROVISION is a new fallocate() allocation mode that > > > sends a hint to (supported) thinly provisioned block devices to > > > allocate space for the given range of sectors via REQ_OP_PROVISION. > > > > > > Signed-off-by: Sarthak Kukreti > > > --- > > > block/fops.c | 7 ++++++- > > > include/linux/falloc.h | 3 ++- > > > include/uapi/linux/falloc.h | 8 ++++++++ > > > 3 files changed, 16 insertions(+), 2 deletions(-) > > > > > > diff --git a/block/fops.c b/block/fops.c > > > index b90742595317..a436a7596508 100644 > > > --- a/block/fops.c > > > +++ b/block/fops.c > > ... > > > @@ -661,6 +662,10 @@ static long blkdev_fallocate(struct file *file, int mode, loff_t start, > > > error = blkdev_issue_discard(bdev, start >> SECTOR_SHIFT, > > > len >> SECTOR_SHIFT, GFP_KERNEL); > > > break; > > > + case FALLOC_FL_PROVISION: > > > + error = blkdev_issue_provision(bdev, start >> SECTOR_SHIFT, > > > + len >> SECTOR_SHIFT, GFP_KERNEL); > > > + break; > > > default: > > > error = -EOPNOTSUPP; > > > } > > > > Hi Sarthak, > > > > Neat mechanism.. I played with something very similar in the past (that > > was much more crudely hacked up to target dm-thin) to allow filesystems > > to request a thinly provisioned device to allocate blocks and try to do > > a better job of avoiding inactivation when overprovisioned. > > > > One thing I'm a little curious about here.. what's the need for a new > > fallocate mode? On a cursory glance, the provision mode looks fairly > > analogous to normal (mode == 0) allocation mode with the exception of > > sending the request down to the bdev. blkdev_fallocate() already maps > > some of the logical falloc modes (i.e. punch hole, zero range) to > > sending write sames or discards, etc., and it doesn't currently look > > like it supports allocation mode, so could it not map such requests to > > the underlying REQ_OP_PROVISION op? > > > > I guess the difference would be at the filesystem level where we'd > > probably need to rely on a mount option or some such to control whether > > traditional fallocate issues provision ops (like you've implemented for > > ext4) vs. the specific falloc command, but that seems fairly consistent > > with historical punch hole/discard behavior too. Hm? You might want to > > cc linux-fsdevel in future posts in any event to get some more feedback > > on how other filesystems might want to interact with such a thing. > > > Thanks for the feedback! > Argh, I completely forgot that I should add linux-fsdevel. Let me > re-send this with linux-fsdevel cc'd > > There's a slight distinction is that the current filesystem-level > controls are usually for default handling, but userspace can still > call the relevant functions manually if they need to. For example, for > ext4, the 'discard' mount option dictates whether free blocks are > discarded, but it doesn't set the policy to allow/disallow userspace > from manually punching holes into files even if the mount opt is > 'nodiscard'. FALLOC_FL_PROVISION is similar in that regard; it adds a > manual mechanism for users to provision the files' extents, that is > separate from the filesystems' default handling of provisioning files. > What I'm trying to understand is why not let blkdev_fallocate() issue a provision based on the default mode (i.e. mode == 0) of fallocate(), which is already defined to mean "perform allocation?" It currently issues discards or write zeroes based on variants of FALLOC_FL_PUNCH_HOLE without the need for a separate FALLOC_FL_DISCARD mode, for example. Brian > > BTW another thing that might be useful wrt to dm-thin is to support > > FALLOC_FL_UNSHARE. I.e., it looks like the previous dm-thin patch only > > checks that blocks are allocated, but not whether those blocks are > > shared (re: lookup_result.shared). It might be useful to do the COW in > > such cases if the caller passes down a REQ_UNSHARE or some such flag. > > > That's an interesting idea! There's a few more things on the TODO list > for this patch series but I think we can follow up with a patch to > handle that as well. > > Sarthak > > > Brian > > > > > diff --git a/include/linux/falloc.h b/include/linux/falloc.h > > > index f3f0b97b1675..a0e506255b20 100644 > > > --- a/include/linux/falloc.h > > > +++ b/include/linux/falloc.h > > > @@ -30,7 +30,8 @@ struct space_resv { > > > FALLOC_FL_COLLAPSE_RANGE | \ > > > FALLOC_FL_ZERO_RANGE | \ > > > FALLOC_FL_INSERT_RANGE | \ > > > - FALLOC_FL_UNSHARE_RANGE) > > > + FALLOC_FL_UNSHARE_RANGE | \ > > > + FALLOC_FL_PROVISION) > > > > > > /* on ia32 l_start is on a 32-bit boundary */ > > > #if defined(CONFIG_X86_64) > > > diff --git a/include/uapi/linux/falloc.h b/include/uapi/linux/falloc.h > > > index 51398fa57f6c..2d323d113eed 100644 > > > --- a/include/uapi/linux/falloc.h > > > +++ b/include/uapi/linux/falloc.h > > > @@ -77,4 +77,12 @@ > > > */ > > > #define FALLOC_FL_UNSHARE_RANGE 0x40 > > > > > > +/* > > > + * FALLOC_FL_PROVISION acts as a hint for thinly provisioned devices to allocate > > > + * blocks for the range/EOF. > > > + * > > > + * FALLOC_FL_PROVISION can only be used with allocate-mode fallocate. > > > + */ > > > +#define FALLOC_FL_PROVISION 0x80 > > > + > > > #endif /* _UAPI_FALLOC_H_ */ > > > -- > > > 2.31.0 > > > > > >