Re: [f2fs-dev] [PATCH 2/2] f2fs: refactor read path to allow multiple postprocessing steps

[Chao Yu <yuchao0@huawei.com>]

Hi Eric,

On 2018/4/18 1:42, Eric Biggers wrote:
> Hi Chao,
> 
> On Tue, Apr 17, 2018 at 05:13:12PM +0800, Chao Yu wrote:
>>> +
>>> +static void bio_post_read_processing(struct bio_post_read_ctx *ctx);
>>> +
>>> +static void decrypt_work(struct work_struct *work)
>>> +{
>>> +	struct bio_post_read_ctx *ctx =
>>> +		container_of(work, struct bio_post_read_ctx, work);
>>> +
>>> +	fscrypt_decrypt_bio(ctx->bio);
>>> +
>>> +	bio_post_read_processing(ctx);
>>> +}
>>> +
>>> +static void bio_post_read_processing(struct bio_post_read_ctx *ctx)
>>> +{
>>> +	switch (++ctx->cur_step) {
>>> +	case STEP_DECRYPT:
>>> +		if (ctx->enabled_steps & (1 << STEP_DECRYPT)) {
>>> +			INIT_WORK(&ctx->work, decrypt_work);
>>> +			fscrypt_enqueue_decrypt_work(&ctx->work);
>>> +			return;
>>> +		}
>>> +		ctx->cur_step++;
>>> +		/* fall-through */
>>> +	default:
>>> +		__read_end_io(ctx->bio);
>>> +	}
>>
>> How about introducing __bio_post_read_processing()
>>
>> switch (step) {
>> case STEP_DECRYPT:
>> 	...
>> 	break;
>> case STEP_COMPRESS:
>> 	...
>> 	break;
>> case STEP_GENERIC:
>> 	__read_end_io;
>> 	break;
>> ...
>> }
>>
>> Then we can customize flexible read processes like:
>>
>> bio_post_read_processing()
>> {
>> 	if (encrypt_enabled)
>> 		__bio_post_read_processing(, STEP_DECRYPT);
>> 	if (compress_enabled)
>> 		__bio_post_read_processing(, STEP_COMPRESS);
>> 	__bio_post_read_processing(, STEP_GENERIC);
>> }
>>
>> Or other flow.
> 
> If I understand correctly, you're suggesting that all the steps be done in a
> single workqueue item?  The problem with that is that the verity work will

Yup,

> require I/O to the file to read hashes, which may need STEP_DECRYPT.  Hence,
> decryption and verity will need separate workqueues.

For decryption and verity, the needs separated data, I agree that we can not
merge the work into one workqueue.

As you mentioned in commit message, it can be used by compression later, so I
just thought that for decryption and decompression, maybe we can do those work
sequentially in one workqueue?

> 
>>> @@ -481,29 +537,33 @@ static struct bio *f2fs_grab_read_bio(struct inode *inode, block_t blkaddr,
>>>  							 unsigned nr_pages)
>>>  {
>>>  	struct f2fs_sb_info *sbi = F2FS_I_SB(inode);
>>> -	struct fscrypt_ctx *ctx = NULL;
>>>  	struct bio *bio;
>>> -
>>> -	if (f2fs_encrypted_file(inode)) {
>>> -		ctx = fscrypt_get_ctx(inode, GFP_NOFS);
>>> -		if (IS_ERR(ctx))
>>> -			return ERR_CAST(ctx);
>>> -
>>> -		/* wait the page to be moved by cleaning */
>>> -		f2fs_wait_on_block_writeback(sbi, blkaddr);
>>> -	}
>>> +	struct bio_post_read_ctx *ctx;
>>> +	unsigned int post_read_steps = 0;
>>>  
>>>  	bio = f2fs_bio_alloc(sbi, min_t(int, nr_pages, BIO_MAX_PAGES), false);
>>> -	if (!bio) {
>>> -		if (ctx)
>>> -			fscrypt_release_ctx(ctx);
>>> +	if (!bio)
>>>  		return ERR_PTR(-ENOMEM);
>>> -	}
>>>  	f2fs_target_device(sbi, blkaddr, bio);
>>>  	bio->bi_end_io = f2fs_read_end_io;
>>> -	bio->bi_private = ctx;
>>
>> bio->bi_private = NULL;
>>
> 
> I don't see why.  ->bi_private is NULL by default.

As we will check bi_private in read_end_io anyway, if it is not NULL, we will
parse it as an ctx, am I missing something?

Thanks,

> 
>>> +	bio_post_read_ctx_pool =
>>> +		mempool_create_slab_pool(128, bio_post_read_ctx_cache);
>>
>> #define MAX_POST_READ_CACHE_SIZE	128
>>
> 
> Yes, that makes sense.
> 
> - Eric
> 
> .
>