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Message-ID: <ZzcuaYVuFuhknNs_@localhost.localdomain>
Date: Fri, 15 Nov 2024 19:20:09 +0800
From: Long Li <leo.lilong@...wei.com>
To: Dave Chinner <dchinner@...hat.com>
CC: John Garry <john.g.garry@...cle.com>, Dave Chinner <david@...morbit.com>,
Ritesh Harjani <ritesh.list@...il.com>, <chandan.babu@...cle.com>,
<djwong@...nel.org>, <hch@....de>, <viro@...iv.linux.org.uk>,
<brauner@...nel.org>, <jack@...e.cz>, <linux-xfs@...r.kernel.org>,
<linux-kernel@...r.kernel.org>, <linux-fsdevel@...r.kernel.org>,
<catherine.hoang@...cle.com>, <martin.petersen@...cle.com>
Subject: Re: [PATCH v4 00/14] forcealign for xfs
On Fri, Nov 15, 2024 at 07:07:53AM +1100, Dave Chinner wrote:
> On Thu, Nov 14, 2024 at 08:48:21PM +0800, Long Li wrote:
> > On Wed, Sep 18, 2024 at 11:12:47AM +0100, John Garry wrote:
> > > On 17/09/2024 23:27, Dave Chinner wrote:
> > > > > # xfs_bmap -vvp mnt/file
> > > > > mnt/file:
> > > > > EXT: FILE-OFFSET BLOCK-RANGE AG AG-OFFSET TOTAL FLAGS
> > > > > 0: [0..15]: 384..399 0 (384..399) 16 010000
> > > > > 1: [16..31]: 400..415 0 (400..415) 16 000000
> > > > > 2: [32..127]: 416..511 0 (416..511) 96 010000
> > > > > 3: [128..255]: 256..383 0 (256..383) 128 000000
> > > > > FLAG Values:
> > > > > 0010000 Unwritten preallocated extent
> > > > >
> > > > > Here we have unaligned extents wrt extsize.
> > > > >
> > > > > The sub-alloc unit zeroing would solve that - is that what you would still
> > > > > advocate (to solve that issue)?
> > > > Yes, I thought that was already implemented for force-align with the
> > > > DIO code via the extsize zero-around changes in the iomap code. Why
> > > > isn't that zero-around code ensuring the correct extent layout here?
> > >
> > > I just have not included the extsize zero-around changes here. They were
> > > just grouped with the atomic writes support, as they were added specifically
> > > for the atomic writes support. Indeed - to me at least - it is strange that
> > > the DIO code changes are required for XFS forcealign implementation. And,
> > > even if we use extsize zero-around changes for DIO path, what about buffered
> > > IO?
> >
> >
> > I've been reviewing and testing the XFS atomic write patch series. Since
> > there haven't been any new responses to the previous discussions on this
> > issue, I'd like to inquire about the buffered IO problem with force-aligned
> > files, which is a scenario we might encounter.
> >
> > Consider a case where the file supports force-alignment with a 64K extent size,
> > and the system page size is 4K. Take the following commands as an example:
> >
> > xfs_io -c "pwrite 64k 64k" mnt/file
> > xfs_io -c "pwrite 8k 8k" mnt/file
> >
> > If unaligned unwritten extents are not permitted, we need to zero out the
> > sub-allocation units for ranges [0, 8K] and [16K, 64K] to prevent stale
> > data. While this can be handled relatively easily in direct I/O scenarios,
> > it presents significant challenges in buffered I/O operations. The main
> > difficulty arises because the extent size (64K) is larger than the page
> > size (4K), and our current code base has substantial limitations in handling
> > such cases.
> >
> > Any thoughts on this?
>
> Large folios in the page cache solve this problem. i.e. it's the
> same problem that block size > page size support had to solve.
>
>
Thanks for your reply, it cleared up my confusion. So maybe we need
to set a minimum folio order for force-aligned inodes, just
like Large block sizes (LBS).
Thanks,
Long Li
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