linux-kernel - Re: [PATCH v2] f2fs: get rid of buffer

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Message-ID: <e56b6166-1bb4-411a-a701-51bf452d2369@kernel.org>
Date: Fri, 7 Jun 2024 22:10:52 +0800
From: Chao Yu <chao@...nel.org>
To: Matthew Wilcox <willy@...radead.org>
Cc: jaegeuk@...nel.org, linux-f2fs-devel@...ts.sourceforge.net,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] f2fs: get rid of buffer_head use

On 2024/6/7 21:33, Matthew Wilcox wrote:
> On Fri, Jun 07, 2024 at 06:18:29PM +0800, Chao Yu wrote:
>> @@ -1990,6 +1989,12 @@ static inline struct f2fs_super_block *F2FS_RAW_SUPER(struct f2fs_sb_info *sbi)
>>   	return (struct f2fs_super_block *)(sbi->raw_super);
>>   }
>>   
>> +static inline struct f2fs_super_block *F2FS_SUPER_BLOCK(struct folio *folio)
>> +{
>> +	return (struct f2fs_super_block *)(page_address(folio_page(folio, 0)) +
>> +							F2FS_SUPER_OFFSET);
>> +}
> 
> This assumes that the superblock is in the first page of the folio.
> That's not necessarily guaranteed; let's say you have a 64KiB folio
> that covers the start of the bdev.

Oh, I missed to add large folio support in this version.

For the case: page size is 4KiB, and folio size is 64KiB,
read_mapping_folio(mapping, 0, NULL) and read_mapping_folio(mapping,
1, NULL) will return the same #0 folio, right?

> 
> I don't quite know how to write this because f2fs defines its block size
> in terms of PAGE_SIZE, which just seems like nonsense to me.  If you
> format a filesystem on a 16KiB PAGE_SIZE machine and then try to mount
> it on a machine with a 4KiB PAGE_SIZE, it's going to go horribly wrong.

f2fs will check on-disk block size w/ PAGE_SIZE in sanity_check_raw_super()
as below:

	/* only support block_size equals to PAGE_SIZE */
	if (le32_to_cpu(raw_super->log_blocksize) != F2FS_BLKSIZE_BITS) {
		f2fs_info(sbi, "Invalid log_blocksize (%u), supports only %u",
			  le32_to_cpu(raw_super->log_blocksize),
			  F2FS_BLKSIZE_BITS);
		return -EFSCORRUPTED;
	}

> 
> You'd need to pass in something that indicates whether you're trying to
> access the first or second superblock; there's no way to tell from the
> folio which one it is.
> 
>> +static int __f2fs_commit_super(struct f2fs_sb_info *sbi, struct folio *folio,
>> +								bool update)
>>   {
>> -	lock_buffer(bh);
>> -	if (super)
>> -		memcpy(bh->b_data + F2FS_SUPER_OFFSET, super, sizeof(*super));
>> -	set_buffer_dirty(bh);
>> -	unlock_buffer(bh);
>> -
>> +	struct bio *bio;
>>   	/* it's rare case, we can do fua all the time */
>> -	return __sync_dirty_buffer(bh, REQ_SYNC | REQ_PREFLUSH | REQ_FUA);
>> +	blk_opf_t opf = REQ_OP_WRITE | REQ_SYNC | REQ_PREFLUSH | REQ_FUA;
>> +	int ret;
>> +
>> +	folio_lock(folio);
>> +	folio_wait_writeback(folio);
>> +	if (update)
>> +		memcpy(F2FS_SUPER_BLOCK(folio), F2FS_RAW_SUPER(sbi),
>> +					sizeof(struct f2fs_super_block));
>> +	folio_mark_dirty(folio);
>> +	folio_clear_dirty_for_io(folio);
>> +	folio_start_writeback(folio);
>> +	folio_unlock(folio);
>> +
>> +	bio = bio_alloc(sbi->sb->s_bdev, 1, opf, GFP_NOFS);
>> +
>> +	/* it doesn't need to set crypto context for superblock update */
>> +	bio->bi_iter.bi_sector = SECTOR_FROM_BLOCK(folio_index(folio));
>> +
>> +	if (!bio_add_folio(bio, folio, PAGE_SIZE, 0))
>> +		f2fs_bug_on(sbi, 1);
> 
> Better make that folio_size(folio) to support bs>PS.

Looks better, will update.

Thanks,

>