Re: [PATCH -next RFC 01/14] block: add some bdev apis

Yu Kuai
2023-12-07T02:45:20Z

Hi,

在 2023/12/06 22:58, Matthew Wilcox 写道:
> On Tue, Dec 05, 2023 at 08:37:15PM +0800, Yu Kuai wrote:
>> +struct folio *bdev_read_folio(struct block_device *bdev, pgoff_t index)
>> +{
>> + return read_mapping_folio(bdev->bd_inode->i_mapping, index, NULL);
>> +}
>> +EXPORT_SYMBOL_GPL(bdev_read_folio);
>
> I'm coming to the opinion that 'index' is the wrong parameter here.
> Looking through all the callers of bdev_read_folio() in this patchset,
> they all have a position in bytes, and they all convert it to
> index for this call. The API should probably be:
>
> struct folio *bdev_read_folio(struct block_device *bdev, loff_t pos)
> {
> return read_mapping_folio(bdev->bd_inode->i_mapping,
> pos / PAGE_SIZE, NULL);
> }

Thanks for reviewing this patchset! Okay, I'll convert to pass in "pos"
in v2.
>
> ... and at some point, we'll get round to converting read_mapping_folio()
> to take its argument in loff_t.
>
> Similiarly for these two APIs:
>
>> +struct folio *bdev_read_folio_gfp(struct block_device *bdev, pgoff_t index,
>> + gfp_t gfp)
>> +struct folio *bdev_get_folio(struct block_device *bdev, pgoff_t index)
>
>> +struct folio *bdev_find_or_create_folio(struct block_device *bdev,
>> + pgoff_t index, gfp_t gfp)
>> +{
>> + return __filemap_get_folio(bdev->bd_inode->i_mapping, index,
>> + FGP_LOCK | FGP_ACCESSED | FGP_CREAT, gfp);
>> +}
>> +EXPORT_SYMBOL_GPL(bdev_find_or_create_folio);
>
> This one probably shouldn't exist. I've been converting callers of
> find_or_create_page() to call __filemap_get_folio; I suspect we
> should expose a __bdev_get_folio and have the callers use the FGP
> arguments directly, but I'm open to other opinions here.

If nobody against this, I will expose single __bdev_get_folio() to use
in v2.
>
>> +void bdev_sync_readahead(struct block_device *bdev, struct file_ra_state *ra,
>> + struct file *file, pgoff_t index,
>> + unsigned long req_count)
>> +{
>> + struct file_ra_state tmp_ra = {};
>> +
>> + if (!ra) {
>> + ra = &tmp_ra;
>> + file_ra_state_init(ra, bdev->bd_inode->i_mapping);
>> + }
>> + page_cache_sync_readahead(bdev->bd_inode->i_mapping, ra, file, index,
>> + req_count);
>> +}
>
> I think the caller should always be passing in a valid file_ra_state.
> It's only cramfs that doesn't have one, and it really should!
> Not entirely sure about the arguments here; part of me says "bytes",
> but this is weird enough to maybe take arguments in pages.

In fact, bdev_sync_readahead() is only called for cramfs and ext4.

For ext4 it's used in ext4_readdir() so there is valid file_ra_state.

Hoever, for cramfs it's used in cramfs_read(), and cramfs_read() is used
for:

1) cramfs_read_folio
2) cramfs_readdir
3) cramfs_lookup
4) cramfs_read_super

Looks like it's easy to pass in valid file_ra_state() for 1) and 2),
however, I don't see an easy way to do this for 3) and 4).

Thanks,
Kuai

>
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