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NFS: Charge unstable writes by request size, not folio size

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nfs_folio_mark_unstable() and nfs_folio_clear_commit() charge and
uncharge NR_WRITEBACK/WB_WRITEBACK by folio_nr_pages(folio) once per
*request* added to or removed from a commit list. This is correct only
when a folio has a single associated request. When pg_test splits a
folio into N sub-folio requests (e.g. pNFS flexfiles striping with a
stripe unit smaller than the folio size, or plain wsize-limited
splitting), each of the N requests independently charges the whole
folio's page count, inflating the accounting by a factor of N per
folio. With large folios and small stripe units this reaches multiple
orders of magnitude: a 2 MiB folio split into 512 4 KiB requests can
charge up to 512x its real size, pushing global dirty+writeback
accounting past the system's dirty threshold and forcing every
buffered writer on the host into the hard-throttle path, including
unrelated in-kernel NFS server threads sharing the box.

Charge each request only for the pages it actually covers.

Fixes: 0c493b5cf1 ("NFS: Convert buffered writes to use folios")
Cc: stable@vger.kernel.org
Signed-off-by: Benjamin Coddington <bcodding@hammerspace.com>
Assisted-By: Claude Sonnet 5 <noreply@anthropic.com>
Signed-off-by: Anna Schumaker <anna.schumaker@hammerspace.com>
This commit is contained in:
Benjamin Coddington
2026-07-07 11:07:16 -04:00
committed by Anna Schumaker
parent 5140f099ec
commit 27934d02cb
3 changed files with 16 additions and 12 deletions
Download Patch File Download Diff File Expand all files Collapse all files

12
fs/nfs/internal.h
View File

@@ -851,17 +851,19 @@ void nfs_super_set_maxbytes(struct super_block *sb, __u64 maxfilesize)
}
/*
* Record the page as unstable (an extra writeback period) and mark its
* inode as dirty.
* Record the request's range as unstable (an extra writeback period) and
* mark its inode as dirty.
*/
static inline void nfs_folio_mark_unstable(struct folio *folio,
static inline void nfs_folio_mark_unstable(struct nfs_page *req,
struct nfs_commit_info *cinfo)
{
struct folio *folio = nfs_page_to_folio(req);
if (folio && !cinfo->dreq) {
struct inode *inode = folio->mapping->host;
long nr = folio_nr_pages(folio);
long nr = DIV_ROUND_UP(req->wb_bytes, PAGE_SIZE);
/* This page is really still in write-back - just that the
/* This range is really still in write-back - just that the
* writeback is happening on the server now.
*/
node_stat_mod_folio(folio, NR_WRITEBACK, nr);

2
fs/nfs/pnfs_nfs.c
View File

@@ -1199,7 +1199,7 @@ pnfs_layout_mark_request_commit(struct nfs_page *req,
nfs_request_add_commit_list_locked(req, list, cinfo);
mutex_unlock(&NFS_I(cinfo->inode)->commit_mutex);
nfs_folio_mark_unstable(nfs_page_to_folio(req), cinfo);
nfs_folio_mark_unstable(req, cinfo);
return;
out_resched:
mutex_unlock(&NFS_I(cinfo->inode)->commit_mutex);

14
fs/nfs/write.c
View File

@@ -807,7 +807,7 @@ nfs_request_add_commit_list(struct nfs_page *req, struct nfs_commit_info *cinfo)
mutex_lock(&NFS_I(cinfo->inode)->commit_mutex);
nfs_request_add_commit_list_locked(req, &cinfo->mds->list, cinfo);
mutex_unlock(&NFS_I(cinfo->inode)->commit_mutex);
nfs_folio_mark_unstable(nfs_page_to_folio(req), cinfo);
nfs_folio_mark_unstable(req, cinfo);
}
EXPORT_SYMBOL_GPL(nfs_request_add_commit_list);
@@ -866,10 +866,12 @@ nfs_mark_request_commit(struct nfs_page *req, struct pnfs_layout_segment *lseg,
nfs_request_add_commit_list(req, cinfo);
}
static void nfs_folio_clear_commit(struct folio *folio)
static void nfs_folio_clear_commit(struct nfs_page *req)
{
struct folio *folio = nfs_page_to_folio(req);
if (folio) {
long nr = folio_nr_pages(folio);
long nr = DIV_ROUND_UP(req->wb_bytes, PAGE_SIZE);
node_stat_mod_folio(folio, NR_WRITEBACK, -nr);
bdi_wb_stat_mod(folio->mapping->host, WB_WRITEBACK, -nr);
@@ -889,7 +891,7 @@ static void nfs_clear_request_commit(struct nfs_commit_info *cinfo,
nfs_request_remove_commit_list(req, cinfo);
}
mutex_unlock(&NFS_I(inode)->commit_mutex);
nfs_folio_clear_commit(nfs_page_to_folio(req));
nfs_folio_clear_commit(req);
}
}
@@ -1741,7 +1743,7 @@ void nfs_retry_commit(struct list_head *page_list,
req = nfs_list_entry(page_list->next);
nfs_list_remove_request(req);
nfs_mark_request_commit(req, lseg, cinfo, ds_commit_idx);
nfs_folio_clear_commit(nfs_page_to_folio(req));
nfs_folio_clear_commit(req);
nfs_unlock_and_release_request(req);
}
}
@@ -1813,7 +1815,7 @@ static void nfs_commit_release_pages(struct nfs_commit_data *data)
req = nfs_list_entry(data->pages.next);
nfs_list_remove_request(req);
folio = nfs_page_to_folio(req);
nfs_folio_clear_commit(folio);
nfs_folio_clear_commit(req);
dprintk("NFS: commit (%s/%llu %d@%lld)",
nfs_req_openctx(req)->dentry->d_sb->s_id,