Re: [PATCH v5 07/11] iomap: fix iomap_dio_zero() for fs bs > system page size

[Christoph Hellwig <hch@infradead.org>]

On Tue, May 07, 2024 at 05:00:28PM +0100, Matthew Wilcox wrote:
> If the len is more than PAGE_SIZE * BIO_MAX_VECS, __bio_add_page()
> will fail silently.  I hate this interface.

No, it won't.  You can pass an arbitray len to it.

> 
> You should be doing something like ...
> 
> 	while (len) {
> 		unsigned int io_len = min_t(unsigned int, len, PAGE_SIZE);
> 
> 		while (!bio || bio_add_page() < io_len) {
> 			if (bio)
> 				iomap_dio_submit_bio(iter, dio, bio, pos);
> 			bio = iomap_dio_alloc_bio(iter, dio, BIO_MAX_VECS,
> 					REQ_OP_WRITE | REQ_SYNC | REQ_IDLE);
> 		 	fscrypt_set_bio_crypt_ctx(bio, inode,
> 					pos >> inode->i_blkbits, GFP_KERNEL);
> 		}
> 	}

Wee, no.  The right way is:

	bio = iomap_dio_alloc_bio(iter, dio, 1,
			REQ_OP_WRITE | REQ_SYNC | REQ_IDLE);
	__bio_add_page(bio, page, len, 0);

	fscrypt_set_bio_crypt_ctx(bio, inode,
			pos >> inode->i_blkbits, GFP_KERNEL);

(or even better the folio version)