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Date:   Wed, 21 Sep 2022 11:39:15 -0400
From:   Brian Foster <bfoster@redhat.com>
To:     Sarthak Kukreti <sarthakkukreti@chromium.org>
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 <axboe@kernel.dk>,
        "Michael S . Tsirkin" <mst@redhat.com>,
        Jason Wang <jasowang@redhat.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Stefan Hajnoczi <stefanha@redhat.com>,
        Alasdair Kergon <agk@redhat.com>,
        Mike Snitzer <snitzer@kernel.org>,
        Theodore Ts'o <tytso@mit.edu>,
        Andreas Dilger <adilger.kernel@dilger.ca>,
        Bart Van Assche <bvanassche@google.com>,
        Daniil Lunev <dlunev@google.com>,
        Evan Green <evgreen@google.com>,
        Gwendal Grignou <gwendal@google.com>
Subject: Re: [PATCH RFC 4/8] fs: Introduce FALLOC_FL_PROVISION
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On Fri, Sep 16, 2022 at 02:02:31PM -0700, Sarthak Kukreti wrote:
> On Fri, Sep 16, 2022 at 4:56 AM Brian Foster <bfoster@redhat.com> wrote:
> >
> > On Thu, Sep 15, 2022 at 09:48:22AM -0700, Sarthak Kukreti wrote:
> > > From: Sarthak Kukreti <sarthakkukreti@chromium.org>
> > >
> > > 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 <sarthakkukreti@chromium.org>
> > > ---
> > >  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
> > >
> >
>