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Message-ID: <4f26b365-bca1-4ca7-99b7-f4b80cff26be@gmail.com>
Date: Fri, 16 May 2025 21:50:16 +0800
From: Sheng Yong <shengyong2021@...il.com>
To: Gao Xiang <hsiangkao@...ux.alibaba.com>, xiang@...nel.org,
chao@...nel.org, zbestahu@...il.com, jefflexu@...ux.alibaba.com,
dhavale@...gle.com, lihongbo22@...wei.com
Cc: linux-erofs@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
Sheng Yong <shengyong1@...omi.com>, Wang Shuai <wangshuai12@...omi.com>
Subject: Re: [PATCH v6] erofs: add 'fsoffset' mount option for file-backed &
bdev-based mounts
Hi Xiang,
On 5/16/25 17:15, Gao Xiang wrote:
> Hi Yong,
>
> On 2025/5/16 17:00, Sheng Yong wrote:
>
> ...
>
>> diff --git a/Documentation/filesystems/erofs.rst b/Documentation/
>> filesystems/erofs.rst
>> index c293f8e37468..b24cb0d5d4d6 100644
>> --- a/Documentation/filesystems/erofs.rst
>> +++ b/Documentation/filesystems/erofs.rst
>> @@ -128,6 +128,7 @@ device=%s Specify a path to an extra
>> device to be used together.
>> fsid=%s Specify a filesystem image ID for Fscache
>> back-end.
>> domain_id=%s Specify a domain ID in fscache mode so that
>> different images
>> with the same blobs under a given domain ID
>> can share storage.
>> +fsoffset=%lu Specify image offset for file-backed or bdev-
>> based mounts.
>
> Maybe document it as:
> fsoffset=%lu Specify filesystem offset for the primary device.
OK, this makes sense. But if we need to handle offset for extra devices,
we might need to use an array or a string to temporarily store the values.
This is because devices are not initialized during parsing parameters.
And set `off' for each extra device during scan devices.
For now, I prefer to add fsoffset for the primary device only. I think
the primary device which has an offset is the more generic case.
>
> Since I'm not sure if we need later
> fsoffset=%lu,[%lu,...] Specify filesystem offset for all devices.
>
>> ===================
>> =========================================================
>> Sysfs Entries
>> diff --git a/fs/erofs/data.c b/fs/erofs/data.c
>> index 2409d2ab0c28..599a44d5d782 100644
>> --- a/fs/erofs/data.c
>> +++ b/fs/erofs/data.c
>> @@ -27,9 +27,12 @@ void erofs_put_metabuf(struct erofs_buf *buf)
>> void *erofs_bread(struct erofs_buf *buf, erofs_off_t offset, bool
>> need_kmap)
>> {
>> - pgoff_t index = offset >> PAGE_SHIFT;
>> + pgoff_t index;
>
> How about just
> pgoff_t index = (offset + buf->off) >> PAGE_SHIFT;
>
> since it's not complex to break it into two statements..
>
>> struct folio *folio = NULL;
>> + offset += buf->off;
>> + index = offset >> PAGE_SHIFT;
>> +
>> if (buf->page) {
>> folio = page_folio(buf->page);
>> if (folio_file_page(folio, index) != buf->page)
>> @@ -54,6 +57,7 @@ void erofs_init_metabuf(struct erofs_buf *buf,
>> struct super_block *sb)
>> struct erofs_sb_info *sbi = EROFS_SB(sb);
>> buf->file = NULL;
>> + buf->off = sbi->dif0.off;
>> if (erofs_is_fileio_mode(sbi)) {
>> buf->file = sbi->dif0.file; /* some fs like FUSE needs it */
>> buf->mapping = buf->file->f_mapping;
>> @@ -299,7 +303,7 @@ static int erofs_iomap_begin(struct inode *inode,
>> loff_t offset, loff_t length,
>> iomap->private = buf.base;
>> } else {
>> iomap->type = IOMAP_MAPPED;
>> - iomap->addr = mdev.m_pa;
>> + iomap->addr = mdev.m_dif->off + mdev.m_pa;
>
> I mean, could we update erofs_init_device() and then
> `mdev.pa` is already an number added by `mdev.m_dif->off`...
>
> Is it possible? since mdev.pa is already a device-based
> offset.
I think in most cases we can add `off' to mdev.m_pa directly in
erofs_map_dev(). But for readdir, erofs_read_metabuf(mdev.m_pa)
is called after erofs_map_dev() in dir's erofs_iomap_begin().
However, read metabuf needs `off'.
>
>> if (flags & IOMAP_DAX)
>> iomap->addr += mdev.m_dif->dax_part_off;
>> }
>> diff --git a/fs/erofs/fileio.c b/fs/erofs/fileio.c
>> index 60c7cc4c105c..a2c7001ff789 100644
>> --- a/fs/erofs/fileio.c
>> +++ b/fs/erofs/fileio.c
>> @@ -147,7 +147,8 @@ static int erofs_fileio_scan_folio(struct
>> erofs_fileio *io, struct folio *folio)
>> if (err)
>> break;
>> io->rq = erofs_fileio_rq_alloc(&io->dev);
>> - io->rq->bio.bi_iter.bi_sector = io->dev.m_pa >> 9;
>> + io->rq->bio.bi_iter.bi_sector =
>> + (io->dev.m_dif->off + io->dev.m_pa) >> 9;
>
> So we don't need here.
>
>> attached = 0;
>> }unambiguous
>> if (!bio_add_folio(&io->rq->bio, folio, len, cur))
>> diff --git a/fs/erofs/internal.h b/fs/erofs/internal.h
>> index 4ac188d5d894..cd8c738f5eb8 100644
>> --- a/fs/erofs/internal.h
>> +++ b/fs/erofs/internal.h
>> @@ -43,6 +43,7 @@ struct erofs_device_info {
>> char *path;
>> struct erofs_fscache *fscache;
>> struct file *file;
>> + u64 off;
>> struct dax_device *dax_dev;
>> u64 dax_part_off;
>
> Maybe `u64 off, dax_part_off;` here?
>
> Also I'm still not quite sure `off` is unambiguous...
> Maybe `dataoff`? not quite sure.
What about fsoff accroding to fsoffset?
>
>> @@ -199,6 +200,7 @@ enum {
>> struct erofs_buf {
>> struct address_space *mapping;
>> struct file *file;
>> + u64 off;
>> struct page *page;
>> void *base;
>> };
>> diff --git a/fs/erofs/super.c b/fs/erofs/super.c
>> index da6ee7c39290..512877d7d855 100644
>> --- a/fs/erofs/super.c
>> +++ b/fs/erofs/super.c
>> @@ -356,7 +356,7 @@ static void erofs_default_options(struct
>> erofs_sb_info *sbi)
>> enum {
>> Opt_user_xattr, Opt_acl, Opt_cache_strategy, Opt_dax, Opt_dax_enum,
>> - Opt_device, Opt_fsid, Opt_domain_id, Opt_directio,
>> + Opt_device, Opt_fsid, Opt_domain_id, Opt_directio, Opt_fsoffset,
>> };
>> static const struct constant_table erofs_param_cache_strategy[] = {
>> @@ -383,6 +383,7 @@ static const struct fs_parameter_spec
>> erofs_fs_parameters[] = {
>> fsparam_string("fsid", Opt_fsid),
>> fsparam_string("domain_id", Opt_domain_id),
>> fsparam_flag_no("directio", Opt_directio),
>> + fsparam_u64("fsoffset", Opt_fsoffset),
>> {}
>> };
>> @@ -506,6 +507,9 @@ static int erofs_fc_parse_param(struct fs_context
>> *fc,
>> errorfc(fc, "%s option not supported",
>> erofs_fs_parameters[opt].name);
>> #endif
>> break;
>> + case Opt_fsoffset:
>> + sbi->dif0.off = result.uint_64;
>> + break;
>> }
>> return 0;
>> }
>> @@ -599,6 +603,10 @@ static int erofs_fc_fill_super(struct super_block
>> *sb, struct fs_context *fc)
>> &sbi->dif0.dax_part_off, NULL, NULL);
>> }
>> + if (sbi->dif0.off & ((1 << sbi->blkszbits) - 1))
>> + return invalfc(fc, "fsoffset %lld not aligned to block size",
>
> is `sbi->blkszbits` valid here? I think it should be moved down
> to "erofs_read_superblock(sb)".
>
> "fsoffset %llu is not aligned to block size %u",
> sbi->dif0.off, (1 << sbi->blkszbits)
>
>> + sbi->dif0.off);
>
> If fscache doesn't work, we might need to fail out here too.
>
OK, will fix that in the next version.
thanks,
shengyong
>
>
>> +
>> err = erofs_read_superblock(sb);
>> if (err)
>> return err;
>> @@ -947,6 +955,8 @@ static int erofs_show_options(struct seq_file
>> *seq, struct dentry *root)
>> if (sbi->domain_id)
>> seq_printf(seq, ",domain_id=%s", sbi->domain_id);
>> #endif
>> + if (sbi->dif0.off)
>> + seq_printf(seq, ",fsoffset=%lld", sbi->dif0.off);
>> return 0;
>> }
>> diff --git a/fs/erofs/zdata.c b/fs/erofs/zdata.c
>> index b8e6b76c23d5..4f910d7ffb2f 100644
>> --- a/fs/erofs/zdata.c
>> +++ b/fs/erofs/zdata.c
>> @@ -1707,7 +1707,8 @@ static void z_erofs_submit_queue(struct
>> z_erofs_frontend *f,
>> bio = bio_alloc(mdev.m_bdev, BIO_MAX_VECS,
>> REQ_OP_READ, GFP_NOIO);
>> bio->bi_end_io = z_erofs_endio;
>> - bio->bi_iter.bi_sector = cur >> 9;
>> + bio->bi_iter.bi_sector =
>> + (mdev.m_dif->off + cur) >> 9;
>
> So we don't need here as well.
>
> Thanks,
> Gao Xiang
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