[v3,19/19] fs: handle inode->i_version more efficiently

Message ID	20171218151156.14565-20-jlayton@kernel.org (mailing list archive)
State	New, archived
Headers	show Return-Path: <linux-fsdevel-owner@kernel.org> DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 3BE3821928 From: Jeff Layton <jlayton@kernel.org> To: linux-fsdevel@vger.kernel.org Cc: linux-kernel@vger.kernel.org, viro@zeniv.linux.org.uk, linux-nfs@vger.kernel.org, bfields@fieldses.org, neilb@suse.de, jack@suse.de, linux-ext4@vger.kernel.org, tytso@mit.edu, adilger.kernel@dilger.ca, linux-xfs@vger.kernel.org, darrick.wong@oracle.com, david@fromorbit.com, linux-btrfs@vger.kernel.org, clm@fb.com, jbacik@fb.com, dsterba@suse.com, linux-integrity@vger.kernel.org, zohar@linux.vnet.ibm.com, dmitry.kasatkin@gmail.com, linux-afs@lists.infradead.org, dhowells@redhat.com, jaltman@auristor.com Subject: [PATCH v3 19/19] fs: handle inode->i_version more efficiently Date: Mon, 18 Dec 2017 10:11:56 -0500 Message-Id: <20171218151156.14565-20-jlayton@kernel.org> In-Reply-To: <20171218151156.14565-1-jlayton@kernel.org> References: <20171218151156.14565-1-jlayton@kernel.org> Sender: linux-fsdevel-owner@vger.kernel.org Precedence: bulk

diff --git a/include/linux/fs.h b/include/linux/fs.h index 76382c24e9d0..6804d075933e 100644 --- a/include/linux/fs.h +++ b/include/linux/fs.h @@ -639,7 +639,7 @@ struct inode { struct hlist_head i_dentry; struct rcu_head i_rcu; }; - u64 i_version; + atomic64_t i_version; atomic_t i_count; atomic_t i_dio_count; atomic_t i_writecount; diff --git a/include/linux/iversion.h b/include/linux/iversion.h index e08c634779df..a9fbf99709df 100644 --- a/include/linux/iversion.h +++ b/include/linux/iversion.h @@ -5,6 +5,8 @@ #include <linux/fs.h> /* + * The inode->i_version field: + * --------------------------- * The change attribute (i_version) is mandated by NFSv4 and is mostly for * knfsd, but is also used for other purposes (e.g. IMA). The i_version must * appear different to observers if there was a change to the inode's data or @@ -27,86 +29,143 @@ * i_version on namespace changes in directories (mkdir, rmdir, unlink, etc.). * We consider these sorts of filesystems to have a kernel-managed i_version. * + * This implementation uses the low bit in the i_version field as a flag to + * track when the value has been queried. If it has not been queried since it + * was last incremented, we can skip the increment in most cases. + * + * In the event that we're updating the ctime, we will usually go ahead and + * bump the i_version anyway. Since that has to go to stable storage in some + * fashion, we might as well increment it as well. + * + * With this implementation, the value should always appear to observers to + * increase over time if the file has changed. It's recommended to use + * inode_cmp_iversion() helper to compare values. + * * Note that some filesystems (e.g. NFS and AFS) just use the field to store - * a server-provided value (for the most part). For that reason, those + * a server-provided value for the most part. For that reason, those * filesystems do not set SB_I_VERSION. These filesystems are considered to * have a self-managed i_version. + * + * Persistently storing the i_version + * ---------------------------------- + * Queries of the i_version field are not gated on them hitting the backing + * store. It's always possible that the host could crash after allowing + * a query of the value but before it has made it to disk. + * + * To mitigate this problem, filesystems should always use + * inode_set_iversion_queried when loading an existing inode from disk. This + * ensures that the next attempted inode increment will result in the value + * changing. + * + * Storing the value to disk therefore does not count as a query, so those + * filesystems should use inode_peek_iversion to grab the value to be stored. + * There is no need to flag the value as having been queried in that case. */ +/* + * We borrow the lowest bit in the i_version to use as a flag to tell whether + * it has been queried since we last incremented it. If it has, then we must + * increment it on the next change. After that, we can clear the flag and + * avoid incrementing it again until it has again been queried. + */ +#define I_VERSION_QUERIED_SHIFT (1) +#define I_VERSION_QUERIED (1ULL << (I_VERSION_QUERIED_SHIFT - 1)) +#define I_VERSION_INCREMENT (1ULL << I_VERSION_QUERIED_SHIFT) + /** * inode_set_iversion_raw - set i_version to the specified raw value * @inode: inode to set - * @new: new i_version value to set + * @val: new i_version value to set * - * Set @inode's i_version field to @new. This function is for use by + * Set @inode's i_version field to @val. This function is for use by * filesystems that self-manage the i_version. * * For example, the NFS client stores its NFSv4 change attribute in this way, * and the AFS client stores the data_version from the server here. */ static inline void -inode_set_iversion_raw(struct inode *inode, const u64 new) +inode_set_iversion_raw(struct inode *inode, const u64 val) { - inode->i_version = new; + atomic64_set(&inode->i_version, val); } /** * inode_set_iversion - set i_version to a particular value * @inode: inode to set - * @new: new i_version value to set + * @val: new i_version value to set * - * Set @inode's i_version field to @new. This function is for filesystems with - * a kernel-managed i_version. + * Set @inode's i_version field to @val. This function is for filesystems with + * a kernel-managed i_version, for initializing a newly-created inode from + * scratch. * - * For now, this just does the same thing as the _raw variant. + * In this case, we do not set the QUERIED flag since we know that this value + * has never been queried. */ static inline void -inode_set_iversion(struct inode *inode, const u64 new) +inode_set_iversion(struct inode *inode, const u64 val) { - inode_set_iversion_raw(inode, new); + inode_set_iversion_raw(inode, val << I_VERSION_QUERIED_SHIFT); } /** - * inode_set_iversion_queried - set i_version to a particular value and set - * flag to indicate that it has been viewed + * inode_set_iversion_queried - set i_version to a particular value as quereied * @inode: inode to set - * @new: new i_version value to set + * @val: new i_version value to set * - * When loading in an i_version value from a backing store, we typically don't - * know whether it was previously viewed before being stored or not. Thus, we - * must assume that it was, to ensure that any changes will result in the - * value changing. + * Set @inode's i_version field to @val, and flag it for increment on the next + * change. * - * This function will set the inode's i_version, and possibly flag the value - * as if it has already been viewed at least once. + * Filesystems that persistently store the i_version on disk should use this + * when loading an existing inode from disk. * - * For now, this just does what inode_set_iversion does. + * When loading in an i_version value from a backing store, we can't be certain + * that it wasn't previously viewed before being stored. Thus, we must assume + * that it was, to ensure that we don't end up handing out the same value for + * different versions of the same inode. */ static inline void -inode_set_iversion_queried(struct inode *inode, const u64 new) +inode_set_iversion_queried(struct inode *inode, const u64 val) { - inode_set_iversion(inode, new); + inode_set_iversion_raw(inode, (val << I_VERSION_QUERIED_SHIFT) | + I_VERSION_QUERIED); } /** * inode_maybe_inc_iversion - increments i_version * @inode: inode with the i_version that should be updated - * @force: increment the counter even if it's not necessary + * @force: increment the counter even if it's not necessary? * * Every time the inode is modified, the i_version field must be seen to have * changed by any observer. * - * In this implementation, we always increment it after taking the i_lock to - * ensure that we don't race with other incrementors. + * If "force" is set or the QUERIED flag is set, then ensure that we increment + * the value, and clear the queried flag. + * + * In the common case where neither is set, then we can return "false" without + * updating i_version. * - * Returns true if counter was bumped, and false if it wasn't. + * If this function returns false, and no other metadata has changed, then we + * can avoid logging the metadata. */ static inline bool inode_maybe_inc_iversion(struct inode *inode, bool force) { - atomic64_t *ivp = (atomic64_t *)&inode->i_version; + u64 cur, old, new; - atomic64_inc(ivp); + cur = (u64)atomic64_read(&inode->i_version); + for (;;) { + /* If flag is clear then we needn't do anything */ + if (!force && !(cur & I_VERSION_QUERIED)) + return false; + + /* Since lowest bit is flag, add 2 to avoid it */ + new = (cur & ~I_VERSION_QUERIED) + I_VERSION_INCREMENT; + + old = atomic64_cmpxchg(&inode->i_version, cur, new); + if (likely(old == cur)) + break; + cur = old; + } return true; } @@ -124,21 +183,6 @@ inode_inc_iversion(struct inode *inode) inode_maybe_inc_iversion(inode, true); } -/** - * inode_iversion_need_inc - is the i_version in need of being incremented? - * @inode: inode to check - * - * Returns whether the inode->i_version counter needs incrementing on the next - * change. - * - * For now, we assume that it always does. - */ -static inline bool -inode_iversion_need_inc(struct inode *inode) -{ - return true; -} - /** * inode_peek_iversion_raw - grab a "raw" iversion value * @inode: inode from which i_version should be read @@ -153,7 +197,20 @@ inode_iversion_need_inc(struct inode *inode) static inline u64 inode_peek_iversion_raw(const struct inode *inode) { - return inode->i_version; + return atomic64_read(&inode->i_version); +} + +/** + * inode_iversion_need_inc - is the i_version in need of being incremented? + * @inode: inode to check + * + * Returns whether the inode->i_version counter needs incrementing on the next + * change. Just fetch the value and check the QUERIED flag. + */ +static inline bool +inode_iversion_need_inc(struct inode *inode) +{ + return inode_peek_iversion_raw(inode) & I_VERSION_QUERIED; } /** @@ -170,7 +227,7 @@ inode_peek_iversion_raw(const struct inode *inode) static inline u64 inode_peek_iversion(const struct inode *inode) { - return inode_peek_iversion_raw(inode); + return inode_peek_iversion_raw(inode) >> I_VERSION_QUERIED_SHIFT; } /** @@ -182,12 +239,28 @@ inode_peek_iversion(const struct inode *inode) * that a later query of the i_version will result in a different value if * anything has changed. * - * This implementation just does a peek. + * In this implementation, we fetch the current value, set the QUERIED flag and + * then try to swap it into place with a cmpxchg, if it wasn't already set. If + * that fails, we try again with the newly fetched value from the cmpxchg. */ static inline u64 inode_query_iversion(struct inode *inode) { - return inode_peek_iversion(inode); + u64 cur, old, new; + + cur = atomic64_read(&inode->i_version); + for (;;) { + /* If flag is already set, then no need to swap */ + if (cur & I_VERSION_QUERIED) + break; + + new = cur | I_VERSION_QUERIED; + old = atomic64_cmpxchg(&inode->i_version, cur, new); + if (old == cur) + break; + cur = old; + } + return cur >> I_VERSION_QUERIED_SHIFT; } /** @@ -196,11 +269,18 @@ inode_query_iversion(struct inode *inode) * @old: old value to check against its i_version * * Compare an i_version counter with a previous one. Returns 0 if they are - * the same or non-zero if they are different. + * the same, a positive value if the one in the inode appears newer than @old, + * and a negative value if @old appears to be newer than the one in the + * inode. + * + * Note that we don't need to set the QUERIED flag in this case, as the value + * in the inode is not being recorded for later use. */ + static inline s64 inode_cmp_iversion(const struct inode *inode, const u64 old) { - return (s64)inode_peek_iversion(inode) - (s64)old; + return (s64)(inode_peek_iversion_raw(inode) & ~I_VERSION_QUERIED) - + (s64)(old << I_VERSION_QUERIED_SHIFT); } #endif

[v3,19/19] fs: handle inode->i_version more efficiently

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