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bcachefs: mkwrite() now only dirties one page

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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: Kent Overstreet <kent.overstreet@linux.dev>
This commit is contained in:
Kent Overstreet
2025-05-14 21:32:40 -04:00
parent 494d458cfa
commit cbed8287e5
1 changed files with 11 additions and 7 deletions
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18
fs/bcachefs/fs-io-pagecache.c
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@@ -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);