[PATCH RFC 4/4] UNFINISHED mm, fs: use kmem_cache_charge() in path_openat()

[Vlastimil Babka]

This is just an example of using the kmem_cache_charge() API.  I think
it's placed in a place that's applicable for Linus's example [1]
although he mentions do_dentry_open() - I have followed from strace()
showing openat(2) to path_openat() doing the alloc_empty_file().

The idea is that filp_cachep stops being SLAB_ACCOUNT. Allocations that
want to be accounted immediately can use GFP_KERNEL_ACCOUNT. I did that
in alloc_empty_file_noaccount() (despite the contradictory name but the
noaccount refers to something else, right?) as IIUC it's about
kernel-internal opens.

alloc_empty_file() is now not doing the accounting, so I added
kmem_account_file() that calls the new kmem_cache_charge() API.

Why is this unfinished:

- there are other callers of alloc_empty_file() which I didn't adjust so
  they simply became memcg-unaccounted. I haven't investigated for which
  ones it would make also sense to separate the allocation and accounting.
  Maybe alloc_empty_file() would need to get a parameter to control
  this.

- I don't know how to properly unwind the accounting failure case. It
  seems like a new case because when we succeed the open, there's no
  further error path at least in path_openat().

Basically it boils down I'm unfamiliar with VFS so this depends if this
approach is deemed useful enough to finish it.

[1] https://lore.kernel.org/all/CAHk-=whYOOdM7jWy5jdrAm8LxcgCMFyk2bt8fYYvZzM4U-zAQA@xxxxxxxxxxxxxx/

Not-signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
---
 fs/file_table.c | 9 +++++++--
 fs/internal.h   | 1 +
 fs/namei.c      | 4 +++-
 3 files changed, 11 insertions(+), 3 deletions(-)

diff --git a/fs/file_table.c b/fs/file_table.c
index b991f90571b4..6401b6f175ae 100644
--- a/fs/file_table.c
+++ b/fs/file_table.c
@@ -223,6 +223,11 @@ struct file *alloc_empty_file(int flags, const struct cred *cred)
 	return ERR_PTR(-ENFILE);
 }
 
+int kmem_account_file(struct file *f)
+{
+	return kmem_cache_charge(filp_cachep, GFP_KERNEL_ACCOUNT, f);
+}
+
 /*
  * Variant of alloc_empty_file() that doesn't check and modify nr_files.
  *
@@ -234,7 +239,7 @@ struct file *alloc_empty_file_noaccount(int flags, const struct cred *cred)
 	struct file *f;
 	int error;
 
-	f = kmem_cache_zalloc(filp_cachep, GFP_KERNEL);
+	f = kmem_cache_zalloc(filp_cachep, GFP_KERNEL_ACCOUNT);
 	if (unlikely(!f))
 		return ERR_PTR(-ENOMEM);
 
@@ -468,7 +473,7 @@ void __init files_init(void)
 {
 	filp_cachep = kmem_cache_create("filp", sizeof(struct file), 0,
 				SLAB_TYPESAFE_BY_RCU | SLAB_HWCACHE_ALIGN |
-				SLAB_PANIC | SLAB_ACCOUNT, NULL);
+				SLAB_PANIC, NULL);
 	percpu_counter_init(&nr_files, 0, GFP_KERNEL);
 }
 
diff --git a/fs/internal.h b/fs/internal.h
index b67406435fc0..06ada11b71d0 100644
--- a/fs/internal.h
+++ b/fs/internal.h
@@ -96,6 +96,7 @@ extern void chroot_fs_refs(const struct path *, const struct path *);
 struct file *alloc_empty_file(int flags, const struct cred *cred);
 struct file *alloc_empty_file_noaccount(int flags, const struct cred *cred);
 struct file *alloc_empty_backing_file(int flags, const struct cred *cred);
+int kmem_account_file(struct file *file);
 
 static inline void file_put_write_access(struct file *file)
 {
diff --git a/fs/namei.c b/fs/namei.c
index 4e0de939fea1..fcf3f3fcd059 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -3799,8 +3799,10 @@ static struct file *path_openat(struct nameidata *nd,
 		terminate_walk(nd);
 	}
 	if (likely(!error)) {
-		if (likely(file->f_mode & FMODE_OPENED))
+		if (likely(file->f_mode & FMODE_OPENED)) {
+			kmem_account_file(file);
 			return file;
+		}
 		WARN_ON(1);
 		error = -EINVAL;
 	}

-- 
2.44.0