Commit cbed8287 authored by Kent Overstreet's avatar Kent Overstreet
Browse files

bcachefs: mkwrite() now only dirties one page 

Don't dirty the whole folio - fixes write amplification with
applications doing mmaped writes.

https://www.reddit.com/r/bcachefs/comments/1klzcg1/incredible_amounts_of_write_amplification_when/



Signed-off-by: default avatarKent Overstreet <kent.overstreet@linux.dev>
parent 494d458c

fs/bcachefs/fs-io-pagecache.c

+11 −7
Original line number Diff line number Diff line
@@ -605,10 +605,14 @@ vm_fault_t bch2_page_mkwrite(struct vm_fault *vmf) 
	struct address_space *mapping = file->f_mapping;

	struct bch_fs *c = inode->v.i_sb->s_fs_info;

	struct bch2_folio_reservation res;

	unsigned len;

	loff_t isize;

	vm_fault_t ret;



	loff_t file_offset = round_down(vmf->pgoff << PAGE_SHIFT, block_bytes(c));

	unsigned offset = file_offset - folio_pos(folio);

	unsigned len = max(PAGE_SIZE, block_bytes(c));



	BUG_ON(offset + len > folio_size(folio));



	bch2_folio_reservation_init(c, inode, &res);



	sb_start_pagefault(inode->v.i_sb);
@@ -623,24 +627,24 @@ vm_fault_t bch2_page_mkwrite(struct vm_fault *vmf) 
	bch2_pagecache_add_get(inode);



	folio_lock(folio);

	isize = i_size_read(&inode->v);

	u64 isize = i_size_read(&inode->v);



	if (folio->mapping != mapping || folio_pos(folio) >= isize) {

	if (folio->mapping != mapping || file_offset >= isize) {

		folio_unlock(folio);

		ret = VM_FAULT_NOPAGE;

		goto out;

	}



	len = min_t(loff_t, folio_size(folio), isize - folio_pos(folio));

	len = min_t(unsigned, len, isize - file_offset);



	if (bch2_folio_set(c, inode_inum(inode), &folio, 1) ?:

	    bch2_folio_reservation_get(c, inode, folio, &res, 0, len)) {

	    bch2_folio_reservation_get(c, inode, folio, &res, offset, len)) {

		folio_unlock(folio);

		ret = VM_FAULT_SIGBUS;

		goto out;

	}



	bch2_set_folio_dirty(c, inode, folio, &res, 0, len);

	bch2_set_folio_dirty(c, inode, folio, &res, offset, len);

	bch2_folio_reservation_put(c, inode, &res);



	folio_wait_stable(folio);