linux-ext4 - Re: [PATCH -next RFC 01/14] block: add some bdev apis

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Message-ID: <aafabc6e-fd98-f927-44d7-3ef76e2acaf8@huaweicloud.com>
Date: Wed, 6 Dec 2023 14:50:56 +0800
From: Yu Kuai <yukuai1@...weicloud.com>
To: Christoph Hellwig <hch@...radead.org>, Yu Kuai <yukuai1@...weicloud.com>
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Subject: Re: [PATCH -next RFC 01/14] block: add some bdev apis

Hi,

在 2023/12/06 14:14, Christoph Hellwig 写道:
>> +void invalidate_bdev_range(struct block_device *bdev, pgoff_t start,
>> +			   pgoff_t end)
>> +{
>> +	invalidate_mapping_pages(bdev->bd_inode->i_mapping, start, end);
>> +}
>> +EXPORT_SYMBOL_GPL(invalidate_bdev_range);
> 
> All these could probably use kerneldoc comments.

Ok, and thanks for reviewing the patchset!
> 
> For this one I really don't like it existing at all, but we'll have to
> discuss that in the btrfs patch.
> 
>> +loff_t bdev_size(struct block_device *bdev)
>> +{
>> +	loff_t size;
>> +
>> +	spin_lock(&bdev->bd_size_lock);
>> +	size = i_size_read(bdev->bd_inode);
>> +	spin_unlock(&bdev->bd_size_lock);
>> +
>> +	return size;
>> +}
>> +EXPORT_SYMBOL_GPL(bdev_size);
> 
> No need for this one.  The callers can simply use bdev_nr_bytes.

Ok, I'll replace it with bdev_nr_bytes.
> 
>> +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);
>> +
>> +struct folio *bdev_read_folio_gfp(struct block_device *bdev, pgoff_t index,
>> +				  gfp_t gfp)
>> +{
>> +	return mapping_read_folio_gfp(bdev->bd_inode->i_mapping, index, gfp);
>> +}
>> +EXPORT_SYMBOL_GPL(bdev_read_folio_gfp);
> 
> I think we can just drop bdev_read_folio_gfp. Half of the callers simply
> pass GPK_KERNEL, and the other half passes GFP_NOFS and could just use
> memalloc_nofs_save().

I'm a litter confused, so there are 3 use cases:
1) use GFP_USER, default gfp from bdev_alloc.
2) use GFP_KERNEL
3) use GFP_NOFS

I understand that you're suggesting memalloc_nofs_save() to distinguish
2 and 3, but how can I distinguish 1?
> 
>> +void bdev_balance_dirty_pages_ratelimited(struct block_device *bdev)
>> +{
>> +	return balance_dirty_pages_ratelimited(bdev->bd_inode->i_mapping);
>> +}
>> +EXPORT_SYMBOL_GPL(bdev_balance_dirty_pages_ratelimited);
> 
> Hmm, this is just used for block2mtd, and feels a little too low-level
> to me, as block2mtd really should be using the normal fileread/write
> APIs.  I guess we'll have to live with it for now if we want to expedite
> killing off bd_inode.
> 
>> +void bdev_correlate_mapping(struct block_device *bdev,
>> +			    struct address_space *mapping)
>> +{
>> +	mapping->host = bdev->bd_inode;
>> +}
>> +EXPORT_SYMBOL_GPL(bdev_correlate_mapping);
> 
> Maybe associated insted of correlate?  Either way this basically
> fully exposes the bdev inode again :(
> 
>> +gfp_t bdev_gfp_constraint(struct block_device *bdev, gfp_t gfp)
>> +{
>> +	return mapping_gfp_constraint(bdev->bd_inode->i_mapping, gfp);
>> +}
>> +EXPORT_SYMBOL_GPL(bdev_gfp_constraint);
> 
> The right fix here is to:
> 
>   - use memalloc_nofs_save in extet instead of using
>     mapping_gfp_constraint to clear it from the mapping flags
>   - remove __ext4_sb_bread_gfp and just have buffer.c helper that does
>     the right thing (either by changing the calling conventions of an
>     existing one, or adding a new one).

Thanks for the suggestions, but I'm not sure how to do this yet, I must
read more ext4 code.
> 
>> +/*
>> + * The del_gendisk() function uninitializes the disk-specific data
>> + * structures, including the bdi structure, without telling anyone
>> + * else.  Once this happens, any attempt to call mark_buffer_dirty()
>> + * (for example, by ext4_commit_super), will cause a kernel OOPS.
>> + * This is a kludge to prevent these oops until we can put in a proper
>> + * hook in del_gendisk() to inform the VFS and file system layers.
>> + */
>> +int bdev_ejected(struct block_device *bdev)
>> +{
>> +	struct backing_dev_info *bdi = inode_to_bdi(bdev->bd_inode);
>> +
>> +	return bdi->dev == NULL;
>> +}
>> +EXPORT_SYMBOL_GPL(bdev_ejected);
> 
> And this code in ext4 should just go away entirely.  The bdi should
> always be valid for a live bdev for years.
Sounds good, I was confused about this code as well.

> 
>> --- a/block/bio.c
>> +++ b/block/bio.c
>> @@ -1119,6 +1119,7 @@ void bio_add_folio_nofail(struct bio *bio, struct folio *folio, size_t len,
>>   	WARN_ON_ONCE(off > UINT_MAX);
>>   	__bio_add_page(bio, &folio->page, len, off);
>>   }
>> +EXPORT_SYMBOL_GPL(bio_add_folio_nofail);
> 
> How is this realted?  The export is fine, but really should be a
> separate, well-documented commit.

This is used to replace __bio_add_page() in btrfs while converting page
to folio, please let me know if I should keep this, if so, I'll split
this into a new commit.
> 
>>   
>> +static inline u8 block_bits(struct block_device *bdev)
>> +{
>> +	return bdev->bd_inode->i_blkbits;
>> +}
> 
> Not sure we should need this.  i_blkbits comes from the blocksize
> the fs set, so it should have other ways to get at it.

Yes, this is now only used for erofs, and erofs do call
sb_set_blocksize() while initializing, hence it's right there is other
way to get blkbits and this helper is not needed.

Thanks,
Kuai

> .
>