Re: [PATCH RFC 1/3] fs: introduce helper d_path_fast()

Al Viro
2021-05-09T02:21:26Z

On Sat, May 08, 2021 at 01:39:45PM -0700, Linus Torvalds wrote:
> -static int prepend(char **buffer, int *buflen, const char *str, int namelen)
> +struct prepend_buffer {
> + char *ptr;
> + int len;
> +};
> +
> +static int prepend(struct prepend_buffer *b, const char *str, int namelen)
> {
> - *buflen -= namelen;
> - if (*buflen < 0)
> + b->len -= namelen;
> + if (b->len < 0)
> return -ENAMETOOLONG;
> - *buffer -= namelen;
> - memcpy(*buffer, str, namelen);
> + b->ptr -= namelen;
> + memcpy(b->ptr, str, namelen);
> return 0;
> }

OK, that part is pretty obvious - pointers to a couple of objects in the same
stack frame replaced with a single pointer to structure consisting of those
two objects. Might actually be an optimization.

> @@ -35,16 +40,16 @@ static int prepend(char **buffer, int *buflen, const char *str, int namelen)
> *
> * Load acquire is needed to make sure that we see that terminating NUL.
> */
> -static int prepend_name(char **buffer, int *buflen, const struct qstr *name)
> +static int prepend_name(struct prepend_buffer *b, const struct qstr *name)
> {
> const char *dname = smp_load_acquire(&name->name); /* ^^^ */
> u32 dlen = READ_ONCE(name->len);
> char *p;
>
> - *buflen -= dlen + 1;
> - if (*buflen < 0)
> + b->len -= dlen + 1;
> + if (b->len < 0)
> return -ENAMETOOLONG;
> - p = *buffer -= dlen + 1;
> + p = b->ptr -= dlen + 1;
> *p++ = '/';
> while (dlen--) {
> char c = *dname++;

Ditto.

> @@ -55,6 +60,50 @@ static int prepend_name(char **buffer, int *buflen, const struct qstr *name)
> return 0;
> }
>
> +static inline int prepend_entries(struct prepend_buffer *b, const struct path *path, const struct path *root, struct mount *mnt)
> +{
> + struct dentry *dentry = path->dentry;
> + struct vfsmount *vfsmnt = path->mnt;
> +
> + while (dentry != root->dentry || vfsmnt != root->mnt) {
> + int error;
> + struct dentry * parent;
> +
> + if (dentry == vfsmnt->mnt_root || IS_ROOT(dentry)) {
> + struct mount *parent = READ_ONCE(mnt->mnt_parent);
> + struct mnt_namespace *mnt_ns;
> +
> + /* Escaped? */
> + if (dentry != vfsmnt->mnt_root)
> + return 3;
> +
> + /* Global root? */
> + if (mnt != parent) {
> + dentry = READ_ONCE(mnt->mnt_mountpoint);
> + mnt = parent;
> + vfsmnt = &mnt->mnt;
> + continue;
> + }
> + mnt_ns = READ_ONCE(mnt->mnt_ns);
> + /* open-coded is_mounted() to use local mnt_ns */
> + if (!IS_ERR_OR_NULL(mnt_ns) && !is_anon_ns(mnt_ns))
> + return 1; // absolute root
> +
> + return 2; // detached or not attached yet
> + break;
> + }
> + parent = dentry->d_parent;
> + prefetch(parent);
> + error = prepend_name(b, &dentry->d_name);
> + if (error)
> + break;
> +
> + dentry = parent;
> + }
> + return 0;
> +}

See other reply.

> +
> /**
> * prepend_path - Prepend path string to a buffer
> * @path: the dentry/vfsmount to report
> @@ -74,15 +123,12 @@ static int prepend_name(char **buffer, int *buflen, const struct qstr *name)
> */
> static int prepend_path(const struct path *path,
> const struct path *root,
> - char **buffer, int *buflen)
> + struct prepend_buffer *orig)
> {
> - struct dentry *dentry;
> - struct vfsmount *vfsmnt;
> struct mount *mnt;
> int error = 0;
> unsigned seq, m_seq = 0;
> - char *bptr;
> - int blen;
> + struct prepend_buffer b;
>
> rcu_read_lock();
> restart_mnt:
> @@ -90,50 +136,12 @@ static int prepend_path(const struct path *path,
> seq = 0;
> rcu_read_lock();
> restart:
> - bptr = *buffer;
> - blen = *buflen;
> - error = 0;
> - dentry = path->dentry;
> - vfsmnt = path->mnt;
> - mnt = real_mount(vfsmnt);
> + b = *orig;
> + mnt = real_mount(path->mnt);
> read_seqbegin_or_lock(&rename_lock, &seq);
> - while (dentry != root->dentry || vfsmnt != root->mnt) {
> - struct dentry * parent;
>
> - if (dentry == vfsmnt->mnt_root || IS_ROOT(dentry)) {
> - struct mount *parent = READ_ONCE(mnt->mnt_parent);
> - struct mnt_namespace *mnt_ns;
> + error = prepend_entries(&b, path, root, mnt);
>
> - /* Escaped? */
> - if (dentry != vfsmnt->mnt_root) {
> - bptr = *buffer;
> - blen = *buflen;
> - error = 3;
> - break;
> - }
> - /* Global root? */
> - if (mnt != parent) {
> - dentry = READ_ONCE(mnt->mnt_mountpoint);
> - mnt = parent;
> - vfsmnt = &mnt->mnt;
> - continue;
> - }
> - mnt_ns = READ_ONCE(mnt->mnt_ns);
> - /* open-coded is_mounted() to use local mnt_ns */
> - if (!IS_ERR_OR_NULL(mnt_ns) && !is_anon_ns(mnt_ns))
> - error = 1; // absolute root
> - else
> - error = 2; // detached or not attached yet
> - break;
> - }
> - parent = dentry->d_parent;
> - prefetch(parent);
> - error = prepend_name(&bptr, &blen, &dentry->d_name);
> - if (error)
> - break;
> -
> - dentry = parent;
> - }
> if (!(seq & 1))
> rcu_read_unlock();
> if (need_seqretry(&rename_lock, seq)) {
> @@ -150,14 +158,17 @@ static int prepend_path(const struct path *path,
> }
> done_seqretry(&mount_lock, m_seq);
>
> - if (error >= 0 && bptr == *buffer) {
> - if (--blen < 0)
> + // Escaped? No path
> + if (error == 3)
> + b = *orig;
> +
> + if (error >= 0 && b.ptr == orig->ptr) {
> + if (--b.len < 0)
> error = -ENAMETOOLONG;
> else
> - *--bptr = '/';
> + *--b.ptr = '/';
> }
> - *buffer = bptr;
> - *buflen = blen;
> + *orig = b;
> return error;
> }
>
> @@ -181,34 +192,34 @@ char *__d_path(const struct path *path,
> const struct path *root,
> char *buf, int buflen)
> {
> - char *res = buf + buflen;
> + struct prepend_buffer b = { buf + buflen, buflen };
> int error;
>
> - prepend(&res, &buflen, "\0", 1);
> - error = prepend_path(path, root, &res, &buflen);
> + prepend(&b, "\0", 1);
> + error = prepend_path(path, root, &b);

Minor yuck: that should be "", 1 (in the original as well). Same below...
Fairly subtle point: we do *not* need to check for failures here, since
prepend_path() will attempt to produce at least something. And we'll
catch that failure just fine. However, we do depend upon buflen being
non-negative here. If we ever called that (or any other in that family,
really) with buflen == MIN_INT, we'd get seriously unpleasant results.
No such callers exist, thankfully.

> char *simple_dname(struct dentry *dentry, char *buffer, int buflen)
> {
> - char *end = buffer + buflen;
> + struct prepend_buffer b = { buffer + buflen, buflen };
> +
> /* these dentries are never renamed, so d_lock is not needed */
> - if (prepend(&end, &buflen, " (deleted)", 11) ||
> - prepend(&end, &buflen, dentry->d_name.name, dentry->d_name.len) ||
> - prepend(&end, &buflen, "/", 1))
> - end = ERR_PTR(-ENAMETOOLONG);
> - return end;
> + if (prepend(&b, " (deleted)", 11) ||
> + prepend(&b, dentry->d_name.name, dentry->d_name.len) ||
> + prepend(&b, "/", 1))
> + return ERR_PTR(-ENAMETOOLONG);
> + return b.ptr;
> }

Umm... Interesting, especially considering the way dyname_dname() looks like.
char *dynamic_dname(struct dentry *dentry, char *buffer, int buflen,
const char *fmt, ...)
{
va_list args;
char temp[64];
int sz;

va_start(args, fmt);
sz = vsnprintf(temp, sizeof(temp), fmt, args) + 1;
va_end(args);

if (sz > sizeof(temp) || sz > buflen)
return ERR_PTR(-ENAMETOOLONG);

buffer += buflen - sz;
return memcpy(buffer, temp, sz);
}

Looks like there's a piece of prepend() open-coded in it. And
since all ->d_dname() instances are either simple_dname() or end up
with call of dynamic_dname()...

Might make sense to turn that method into
int (*d_dname)(struct dentry *, struct prepend_buffer *);

Followup patch, obviously, but it might be worth looking into.

Another thing that keeps bugging me about prepend_path() is the
set of return values. I mean, 0/1/2/3/-ENAMETOOLONG, and all
except 0 are unlikely? Might as well make that 0/1/2/3/-1, if
not an outright 0/1/2/3/4. And prepend() could return bool, while
we are at it (true - success, false - overflow)...