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Message-ID: <20241006163013.GN21853@frogsfrogsfrogs>
Date: Sun, 6 Oct 2024 09:30:13 -0700
From: "Darrick J. Wong" <djwong@...nel.org>
To: Tang Yizhou <yizhou.tang@...pee.com>
Cc: jack@...e.cz, hch@...radead.org, willy@...radead.org,
akpm@...ux-foundation.org, chandan.babu@...cle.com,
linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
linux-xfs@...r.kernel.org
Subject: Re: [PATCH v2 3/3] xfs: Let the max iomap length be consistent with
the writeback code
On Sun, Oct 06, 2024 at 11:28:49PM +0800, Tang Yizhou wrote:
> From: Tang Yizhou <yizhou.tang@...pee.com>
>
> Since commit 1a12d8bd7b29 ("writeback: scale IO chunk size up to half
> device bandwidth"), macro MAX_WRITEBACK_PAGES has been removed from the
> writeback path. Therefore, the MAX_WRITEBACK_PAGES comments in
> xfs_direct_write_iomap_begin() and xfs_buffered_write_iomap_begin() appear
> outdated.
>
> In addition, Christoph mentioned that the xfs iomap process should be
> similar to writeback, so xfs_max_map_length() was written following the
> logic of writeback_chunk_size().
>
> v2: Thanks for Christoph's advice. Resync with the writeback code.
>
> Signed-off-by: Tang Yizhou <yizhou.tang@...pee.com>
> ---
> fs/fs-writeback.c | 5 ----
> fs/xfs/xfs_iomap.c | 52 ++++++++++++++++++++++++---------------
> include/linux/writeback.h | 5 ++++
> 3 files changed, 37 insertions(+), 25 deletions(-)
>
> diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
> index d8bec3c1bb1f..31c72e207e1b 100644
> --- a/fs/fs-writeback.c
> +++ b/fs/fs-writeback.c
> @@ -31,11 +31,6 @@
> #include <linux/memcontrol.h>
> #include "internal.h"
>
> -/*
> - * 4MB minimal write chunk size
> - */
> -#define MIN_WRITEBACK_PAGES (4096UL >> (PAGE_SHIFT - 10))
> -
> /*
> * Passed into wb_writeback(), essentially a subset of writeback_control
> */
> diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
> index 1e11f48814c0..80f759fa9534 100644
> --- a/fs/xfs/xfs_iomap.c
> +++ b/fs/xfs/xfs_iomap.c
> @@ -4,6 +4,8 @@
> * Copyright (c) 2016-2018 Christoph Hellwig.
> * All Rights Reserved.
> */
> +#include <linux/writeback.h>
> +
> #include "xfs.h"
> #include "xfs_fs.h"
> #include "xfs_shared.h"
> @@ -744,6 +746,34 @@ xfs_ilock_for_iomap(
> return 0;
> }
>
> +/*
> + * We cap the maximum length we map to a sane size to keep the chunks
> + * of work done where somewhat symmetric with the work writeback does.
> + * This is a completely arbitrary number pulled out of thin air as a
> + * best guess for initial testing.
> + *
> + * Following the logic of writeback_chunk_size(), the length will be
> + * rounded to the nearest 4MB boundary.
> + *
> + * Note that the values needs to be less than 32-bits wide until the
> + * lower level functions are updated.
> + */
> +static loff_t
> +xfs_max_map_length(struct inode *inode, loff_t length)
> +{
> + struct bdi_writeback *wb;
> + long pages;
> +
> + spin_lock(&inode->i_lock);
Why's it necessary to hold a spinlock? AFAICT writeback_chunk_size
doesn't hold it.
> + wb = inode_to_wb(wb);
Hmm, it looks like you're trying to cap writes based on storage device
bandwidth instead of a static limit. That could be nifty, but does this
work for a file on the realtime device? Or any device that isn't the
super_block s_bdi?
> + pages = min(wb->avg_write_bandwidth / 2,
> + global_wb_domain.dirty_limit / DIRTY_SCOPE);
> + spin_unlock(&inode->i_lock);
> + pages = round_down(pages + MIN_WRITEBACK_PAGES, MIN_WRITEBACK_PAGES);
> +
> + return min_t(loff_t, length, pages * PAGE_SIZE);
> +}
There's nothing in here that's xfs-specific, shouldn't this be a
fs-writeback.c function for any other filesystem that might want to cap
the size of a write?
--D
> +
> /*
> * Check that the imap we are going to return to the caller spans the entire
> * range that the caller requested for the IO.
> @@ -878,16 +908,7 @@ xfs_direct_write_iomap_begin(
> if (flags & (IOMAP_NOWAIT | IOMAP_OVERWRITE_ONLY))
> goto out_unlock;
>
> - /*
> - * We cap the maximum length we map to a sane size to keep the chunks
> - * of work done where somewhat symmetric with the work writeback does.
> - * This is a completely arbitrary number pulled out of thin air as a
> - * best guess for initial testing.
> - *
> - * Note that the values needs to be less than 32-bits wide until the
> - * lower level functions are updated.
> - */
> - length = min_t(loff_t, length, 1024 * PAGE_SIZE);
> + length = xfs_max_map_length(inode, length);
> end_fsb = xfs_iomap_end_fsb(mp, offset, length);
>
> if (offset + length > XFS_ISIZE(ip))
> @@ -1096,16 +1117,7 @@ xfs_buffered_write_iomap_begin(
> allocfork = XFS_COW_FORK;
> end_fsb = imap.br_startoff + imap.br_blockcount;
> } else {
> - /*
> - * We cap the maximum length we map here to MAX_WRITEBACK_PAGES
> - * pages to keep the chunks of work done where somewhat
> - * symmetric with the work writeback does. This is a completely
> - * arbitrary number pulled out of thin air.
> - *
> - * Note that the values needs to be less than 32-bits wide until
> - * the lower level functions are updated.
> - */
> - count = min_t(loff_t, count, 1024 * PAGE_SIZE);
> + count = xfs_max_map_length(inode, count);
> end_fsb = xfs_iomap_end_fsb(mp, offset, count);
>
> if (xfs_is_always_cow_inode(ip))
> diff --git a/include/linux/writeback.h b/include/linux/writeback.h
> index d6db822e4bb3..657bc4dd22d0 100644
> --- a/include/linux/writeback.h
> +++ b/include/linux/writeback.h
> @@ -17,6 +17,11 @@ struct bio;
>
> DECLARE_PER_CPU(int, dirty_throttle_leaks);
>
> +/*
> + * 4MB minimal write chunk size
> + */
> +#define MIN_WRITEBACK_PAGES (4096UL >> (PAGE_SHIFT - 10))
> +
> /*
> * The global dirty threshold is normally equal to the global dirty limit,
> * except when the system suddenly allocates a lot of anonymous memory and
> --
> 2.25.1
>
>
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