From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from kylie.crudebyte.com (kylie.crudebyte.com [5.189.157.229]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 98B2833BBD9; Wed, 11 Feb 2026 16:17:59 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=5.189.157.229 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1770826681; cv=none; b=VM87WObT5pGEYwgzFSatJIT1HD4OHv6iE6DVVtx+Pjhz33qGt2Nfod9UkBSDKINghvy5Jf/UG60bfhNgQQTv5tlFUR/RHIjAaxolBhMijpwOOgU1pFMxrStQkd94v800MxiPr6+3cTfIpDgewoIiRP28NdAgg5KY6mOiA9UIoQo= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1770826681; c=relaxed/simple; bh=8JobU68D8bC+YDQRkZ3rcmSbXAfjGXnoqq1Ohr18ESM=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version:Content-Type; b=Z5NZyP3lPPKmRUMB6Kbkw/h8NGltpdXdRfKVBFLE7fukkZS9b/cFkCWlFwczteQW7CqcPJfJg96tKNWAZKyzGI2GRNv876K0SK0z/u2ZGIduno0tTNRU8VIO/PuZgJQjIs/wyGOFTMnS80+UEbTSJ230Pa0xwz8hrPXU7Us3E6A= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=crudebyte.com; spf=pass smtp.mailfrom=crudebyte.com; dkim=pass (4096-bit key) header.d=crudebyte.com header.i=@crudebyte.com header.b=hNkl25bH; arc=none smtp.client-ip=5.189.157.229 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=crudebyte.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=crudebyte.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (4096-bit key) header.d=crudebyte.com header.i=@crudebyte.com header.b="hNkl25bH" DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=crudebyte.com; s=kylie; h=Content-Type:Content-Transfer-Encoding: MIME-Version:References:In-Reply-To:Message-ID:Date:Subject:Cc:To:From: Content-ID:Content-Description; bh=OXkFkqt1aOozaRNFCPcreuHds3KfkDW81hEGdpAFSJo=; b=hNkl25bH5JF4JqxtFRjX1xQ+Zn 2kUI1J+J3U/mZt18zmx+wSRaoCMPvFMdUAIr1375MqpnLMcdC9YZpweuDKM6YPsAA4flb/wv6Ygcl EpTiCvrHmhFNUi0aIl4X70w2xE4x8cpB35hdv7gP9t7acmOMS72fr19G7bituQ1s+gxMzD+2b+aAQ HfkfQrFk+hY60giyQLRFz7R/2nPwXtrFy9AS76bq+Id6T3E6yD/VyTPZ2biUDardgujyfTTcUPXEQ DSGN+07yREV7F+zeJSyk1M6TDnY4CNCNIX4a/tZ2wCKpvOj80GzzIYLwUY5XmjvfWtiQB+nu4dMCS M01QT/624QGz2MaJjceNlzq0NjlnNtxRHsxkUVyPPLZhL3MicVGL7ZGqsTnnQWg9gYVraYeA7X8Xm gZCfVtzu0vAPqjDSkrYMmORI79R4DkGDgief28VbGrURndHPjMMDfLxnw21BC68d44zkKx0EJ3tap FeaEKDUce4H6ECT9muwkBpVgaSjKxKmM0U8/OoD3DwpUr08/A7xJOt2EFVhMT9DRD4e0l6mMewVQK P9uPwwODRYOfpF+MjMw9yjK9s7kuoVWwlFm7xGrKPJ0YjYO3d0JFQIOMr88/Tz7ZXgIzX22g3c7dd HR5O79ZA75k2vJfA2JdCuiwEhQ8Yf2Sgxiehq0WFE=; From: Christian Schoenebeck To: v9fs@lists.linux.dev, Remi Pommarel Cc: linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, Eric Van Hensbergen , Latchesar Ionkov , Dominique Martinet , Remi Pommarel Subject: Re: [PATCH v2 1/3] 9p: Cache negative dentries for lookup performance Date: Wed, 11 Feb 2026 16:49:19 +0100 Message-ID: <10801068.nUPlyArG6x@weasel> In-Reply-To: <51afd44abb72d251e2022fbb4d53dd05a03aeed0.1769013622.git.repk@triplefau.lt> References: <51afd44abb72d251e2022fbb4d53dd05a03aeed0.1769013622.git.repk@triplefau.lt> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 7Bit Content-Type: text/plain; charset="utf-8" On Wednesday, 21 January 2026 20:56:08 CET Remi Pommarel wrote: > Not caching negative dentries can result in poor performance for > workloads that repeatedly look up non-existent paths. Each such > lookup triggers a full 9P transaction with the server, adding > unnecessary overhead. > > A typical example is source compilation, where multiple cc1 processes > are spawned and repeatedly search for the same missing header files > over and over again. > > This change enables caching of negative dentries, so that lookups for > known non-existent paths do not require a full 9P transaction. The > cached negative dentries are retained for a configurable duration > (expressed in milliseconds), as specified by the ndentry_timeout > field in struct v9fs_session_info. If set to -1, negative dentries > are cached indefinitely. > > This optimization reduces lookup overhead and improves performance for > workloads involving frequent access to non-existent paths. > > Signed-off-by: Remi Pommarel > --- > fs/9p/fid.c | 11 +++-- > fs/9p/v9fs.c | 1 + > fs/9p/v9fs.h | 2 + > fs/9p/v9fs_vfs.h | 15 ++++++ > fs/9p/vfs_dentry.c | 105 ++++++++++++++++++++++++++++++++++------ > fs/9p/vfs_inode.c | 7 +-- > fs/9p/vfs_super.c | 1 + > include/net/9p/client.h | 2 + > 8 files changed, 122 insertions(+), 22 deletions(-) > > diff --git a/fs/9p/fid.c b/fs/9p/fid.c > index f84412290a30..76242d450aa7 100644 > --- a/fs/9p/fid.c > +++ b/fs/9p/fid.c > @@ -20,7 +20,9 @@ > > static inline void __add_fid(struct dentry *dentry, struct p9_fid *fid) > { > - hlist_add_head(&fid->dlist, (struct hlist_head *)&dentry->d_fsdata); > + struct v9fs_dentry *v9fs_dentry = to_v9fs_dentry(dentry); > + > + hlist_add_head(&fid->dlist, &v9fs_dentry->head); > } > > > @@ -112,6 +114,7 @@ void v9fs_open_fid_add(struct inode *inode, struct > p9_fid **pfid) > > static struct p9_fid *v9fs_fid_find(struct dentry *dentry, kuid_t uid, int > any) { > + struct v9fs_dentry *v9fs_dentry = to_v9fs_dentry(dentry); > struct p9_fid *fid, *ret; > > p9_debug(P9_DEBUG_VFS, " dentry: %pd (%p) uid %d any %d\n", > @@ -119,11 +122,9 @@ static struct p9_fid *v9fs_fid_find(struct dentry > *dentry, kuid_t uid, int any) any); > ret = NULL; > /* we'll recheck under lock if there's anything to look in */ > - if (dentry->d_fsdata) { > - struct hlist_head *h = (struct hlist_head *)&dentry->d_fsdata; > - > + if (!hlist_empty(&v9fs_dentry->head)) { > spin_lock(&dentry->d_lock); > - hlist_for_each_entry(fid, h, dlist) { > + hlist_for_each_entry(fid, &v9fs_dentry->head, dlist) { > if (any || uid_eq(fid->uid, uid)) { > ret = fid; > p9_fid_get(ret); > diff --git a/fs/9p/v9fs.c b/fs/9p/v9fs.c > index 057487efaaeb..1da7ab186478 100644 > --- a/fs/9p/v9fs.c > +++ b/fs/9p/v9fs.c > @@ -422,6 +422,7 @@ static void v9fs_apply_options(struct v9fs_session_info > *v9ses, v9ses->cache = ctx->session_opts.cache; > v9ses->uid = ctx->session_opts.uid; > v9ses->session_lock_timeout = ctx->session_opts.session_lock_timeout; > + v9ses->ndentry_timeout = ctx->session_opts.ndentry_timeout; > } > > /** > diff --git a/fs/9p/v9fs.h b/fs/9p/v9fs.h > index 6a12445d3858..99d1a0ff3368 100644 > --- a/fs/9p/v9fs.h > +++ b/fs/9p/v9fs.h > @@ -91,6 +91,7 @@ enum p9_cache_bits { > * @debug: debug level > * @afid: authentication handle > * @cache: cache mode of type &p9_cache_bits > + * @ndentry_timeout: Negative dentry lookup cache retention time in ms > * @cachetag: the tag of the cache associated with this session > * @fscache: session cookie associated with FS-Cache > * @uname: string user name to mount hierarchy as > @@ -116,6 +117,7 @@ struct v9fs_session_info { > unsigned short debug; > unsigned int afid; > unsigned int cache; > + unsigned int ndentry_timeout; Why not (signed) long? > #ifdef CONFIG_9P_FSCACHE > char *cachetag; > struct fscache_volume *fscache; > diff --git a/fs/9p/v9fs_vfs.h b/fs/9p/v9fs_vfs.h > index d3aefbec4de6..7e6e8881081c 100644 > --- a/fs/9p/v9fs_vfs.h > +++ b/fs/9p/v9fs_vfs.h > @@ -28,6 +28,19 @@ > /* flags for v9fs_stat2inode() & v9fs_stat2inode_dotl() */ > #define V9FS_STAT2INODE_KEEP_ISIZE 1 > > +/** > + * struct v9fs_dentry - v9fs specific dentry data > + * @head: List of fid associated with this dentry > + * @expire_time: Lookup cache expiration time for negative dentries > + * @rcu: used by kfree_rcu to schedule clean up job > + */ > +struct v9fs_dentry { > + struct hlist_head head; > + u64 expire_time; > + struct rcu_head rcu; > +}; > +#define to_v9fs_dentry(d) ((struct v9fs_dentry *)((d)->d_fsdata)) > + > extern struct file_system_type v9fs_fs_type; > extern const struct address_space_operations v9fs_addr_operations; > extern const struct file_operations v9fs_file_operations; > @@ -35,6 +48,8 @@ extern const struct file_operations > v9fs_file_operations_dotl; extern const struct file_operations > v9fs_dir_operations; > extern const struct file_operations v9fs_dir_operations_dotl; > extern const struct dentry_operations v9fs_dentry_operations; > +extern void v9fs_dentry_refresh(struct dentry *dentry); > +extern void v9fs_dentry_fid_remove(struct dentry *dentry); > extern const struct dentry_operations v9fs_cached_dentry_operations; > extern struct kmem_cache *v9fs_inode_cache; > > diff --git a/fs/9p/vfs_dentry.c b/fs/9p/vfs_dentry.c > index c5bf74d547e8..90291cf0a34b 100644 > --- a/fs/9p/vfs_dentry.c > +++ b/fs/9p/vfs_dentry.c > @@ -23,6 +23,46 @@ > #include "v9fs_vfs.h" > #include "fid.h" > > +/** > + * v9fs_dentry_is_expired - Check if dentry lookup has expired > + * > + * This should be called to know if a negative dentry should be removed > from + * cache. > + * > + * @dentry: dentry in question > + * > + */ > +static bool v9fs_dentry_is_expired(struct dentry const *dentry) > +{ > + struct v9fs_session_info *v9ses = v9fs_dentry2v9ses(dentry); > + struct v9fs_dentry *v9fs_dentry = to_v9fs_dentry(dentry); > + > + if (v9ses->ndentry_timeout == -1) > + return false; > + > + return time_before_eq64(v9fs_dentry->expire_time, get_jiffies_64()); > +} v9fs_negative_dentry_is_expired() ? Or is there a plan to use this for regular dentries, say with cache=loose in future? > + > +/** > + * v9fs_dentry_refresh - Refresh dentry lookup cache timeout > + * > + * This should be called when a look up yields a negative entry. > + * > + * @dentry: dentry in question > + * > + */ > +void v9fs_dentry_refresh(struct dentry *dentry) > +{ > + struct v9fs_session_info *v9ses = v9fs_dentry2v9ses(dentry); > + struct v9fs_dentry *v9fs_dentry = to_v9fs_dentry(dentry); > + > + if (v9ses->ndentry_timeout == -1) > + return; > + > + v9fs_dentry->expire_time = get_jiffies_64() + > + msecs_to_jiffies(v9ses->ndentry_timeout); > +} v9fs_negative_dentry_refresh_timeout() ? > + > /** > * v9fs_cached_dentry_delete - called when dentry refcount equals 0 > * @dentry: dentry in question > @@ -33,20 +73,15 @@ static int v9fs_cached_dentry_delete(const struct dentry > *dentry) p9_debug(P9_DEBUG_VFS, " dentry: %pd (%p)\n", > dentry, dentry); > > - /* Don't cache negative dentries */ > - if (d_really_is_negative(dentry)) > - return 1; > - return 0; > -} > + if (!d_really_is_negative(dentry)) > + return 0; Is it worth a check for v9ses->ndentry_timeout != 0 here? > > -/** > - * v9fs_dentry_release - called when dentry is going to be freed > - * @dentry: dentry that is being release > - * > - */ > + return v9fs_dentry_is_expired(dentry); > +} "... is being released" > > -static void v9fs_dentry_release(struct dentry *dentry) > +static void __v9fs_dentry_fid_remove(struct dentry *dentry) > { > + struct v9fs_dentry *v9fs_dentry = to_v9fs_dentry(dentry); > struct hlist_node *p, *n; > struct hlist_head head; > > @@ -54,13 +89,54 @@ static void v9fs_dentry_release(struct dentry *dentry) > dentry, dentry); > > spin_lock(&dentry->d_lock); > - hlist_move_list((struct hlist_head *)&dentry->d_fsdata, &head); > + hlist_move_list(&v9fs_dentry->head, &head); > spin_unlock(&dentry->d_lock); > > hlist_for_each_safe(p, n, &head) > p9_fid_put(hlist_entry(p, struct p9_fid, dlist)); > } > > +/** > + * v9fs_dentry_fid_remove - Release all dentry's fid > + * @dentry: dentry in question > + * > + */ > +void v9fs_dentry_fid_remove(struct dentry *dentry) > +{ > + __v9fs_dentry_fid_remove(dentry); > +} " ... all dentry's fids" ? > + > +/** > + * v9fs_dentry_init - Initialize v9fs dentry data > + * @dentry: dentry in question > + * > + */ > +static int v9fs_dentry_init(struct dentry *dentry) > +{ > + struct v9fs_dentry *v9fs_dentry = kzalloc(sizeof(*v9fs_dentry), > + GFP_KERNEL); > + > + if (!v9fs_dentry) > + return -ENOMEM; > + > + INIT_HLIST_HEAD(&v9fs_dentry->head); > + dentry->d_fsdata = (void *)v9fs_dentry; > + return 0; > +} > + > +/** > + * v9fs_dentry_release - called when dentry is going to be freed > + * @dentry: dentry that is being release > + * > + */ > +static void v9fs_dentry_release(struct dentry *dentry) > +{ > + struct v9fs_dentry *v9fs_dentry = to_v9fs_dentry(dentry); > + > + __v9fs_dentry_fid_remove(dentry); > + kfree_rcu(v9fs_dentry, rcu); > +} > + > static int __v9fs_lookup_revalidate(struct dentry *dentry, unsigned int > flags) { > struct p9_fid *fid; > @@ -72,7 +148,7 @@ static int __v9fs_lookup_revalidate(struct dentry > *dentry, unsigned int flags) > > inode = d_inode(dentry); > if (!inode) > - goto out_valid; > + return !v9fs_dentry_is_expired(dentry); > > v9inode = V9FS_I(inode); > if (v9inode->cache_validity & V9FS_INO_INVALID_ATTR) { > @@ -112,7 +188,6 @@ static int __v9fs_lookup_revalidate(struct dentry > *dentry, unsigned int flags) return retval; > } > } > -out_valid: > p9_debug(P9_DEBUG_VFS, "dentry: %pd (%p) is valid\n", dentry, dentry); > return 1; > } > @@ -139,12 +214,14 @@ const struct dentry_operations > v9fs_cached_dentry_operations = { .d_revalidate = v9fs_lookup_revalidate, > .d_weak_revalidate = __v9fs_lookup_revalidate, > .d_delete = v9fs_cached_dentry_delete, > + .d_init = v9fs_dentry_init, > .d_release = v9fs_dentry_release, > .d_unalias_trylock = v9fs_dentry_unalias_trylock, > .d_unalias_unlock = v9fs_dentry_unalias_unlock, > }; > > const struct dentry_operations v9fs_dentry_operations = { > + .d_init = v9fs_dentry_init, > .d_release = v9fs_dentry_release, > .d_unalias_trylock = v9fs_dentry_unalias_trylock, > .d_unalias_unlock = v9fs_dentry_unalias_unlock, > diff --git a/fs/9p/vfs_inode.c b/fs/9p/vfs_inode.c > index 0f3189a0a516..a82a71be309b 100644 > --- a/fs/9p/vfs_inode.c > +++ b/fs/9p/vfs_inode.c > @@ -549,7 +549,7 @@ static int v9fs_remove(struct inode *dir, struct dentry > *dentry, int flags) > > /* invalidate all fids associated with dentry */ > /* NOTE: This will not include open fids */ > - dentry->d_op->d_release(dentry); > + v9fs_dentry_fid_remove(dentry); > } > return retval; > } > @@ -732,9 +732,10 @@ struct dentry *v9fs_vfs_lookup(struct inode *dir, > struct dentry *dentry, name = dentry->d_name.name; > fid = p9_client_walk(dfid, 1, &name, 1); > p9_fid_put(dfid); > - if (fid == ERR_PTR(-ENOENT)) > + if (fid == ERR_PTR(-ENOENT)) { > inode = NULL; > - else if (IS_ERR(fid)) > + v9fs_dentry_refresh(dentry); > + } else if (IS_ERR(fid)) > inode = ERR_CAST(fid); > else if (v9ses->cache & (CACHE_META|CACHE_LOOSE)) > inode = v9fs_get_inode_from_fid(v9ses, fid, dir->i_sb); > diff --git a/fs/9p/vfs_super.c b/fs/9p/vfs_super.c > index 315336de6f02..9d9360f9e502 100644 > --- a/fs/9p/vfs_super.c > +++ b/fs/9p/vfs_super.c > @@ -327,6 +327,7 @@ static int v9fs_init_fs_context(struct fs_context *fc) > ctx->session_opts.uid = INVALID_UID; > ctx->session_opts.dfltuid = V9FS_DEFUID; > ctx->session_opts.dfltgid = V9FS_DEFGID; > + ctx->session_opts.ndentry_timeout = 0; > > /* initialize client options */ > ctx->client_opts.proto_version = p9_proto_2000L; > diff --git a/include/net/9p/client.h b/include/net/9p/client.h > index 838a94218b59..3d2483db9259 100644 > --- a/include/net/9p/client.h > +++ b/include/net/9p/client.h > @@ -192,6 +192,7 @@ struct p9_rdma_opts { > * @dfltgid: default numeric groupid to mount hierarchy as > * @uid: if %V9FS_ACCESS_SINGLE, the numeric uid which mounted the > hierarchy * @session_lock_timeout: retry interval for blocking locks > + * @ndentry_timeout: Negative dentry lookup cache retention time in ms > * > * This strucure holds options which are parsed and will be transferred > * to the v9fs_session_info structure when mounted, and therefore largely > @@ -212,6 +213,7 @@ struct p9_session_opts { > kgid_t dfltgid; > kuid_t uid; > long session_lock_timeout; > + unsigned int ndentry_timeout; > }; > > /* Used by mount API to store parsed mount options */