[PATCH v3 00/13] docs: path-lookup: Update pathlookup docs

[PATCH v3 00/13] docs: path-lookup: Update pathlookup docs

[Fox Chen]

The Path lookup is a very complex subject in VFS. The path-lookup
document provides a very detailed guidance to help people understand
how path lookup works in the kernel. This document was originally
written based on three lwn articles five years ago. As times goes by,
some of the content is outdated. This patchset is intended to update
the document to make it more relevant to current codebase.


---
v1: https://lore.kernel.org/lkml/20210126072443.33066-1-foxhlchen@gmail.com/
v2: https://lore.kernel.org/lkml/20210316054727.25655-1-foxhlchen@gmail.com/
 - Fix problems in v1 reviewed by Neil:
   1. In Patch 01 and 02 rewrite a new paragrah to describe step_into()
   2. In Patch 01 instead of changing it to traverse_mounts, remove follow_managed()
   3. In Patch 03 re-telling the story rather than adding notes
   4. In Patch 04 do_open() should be outside of loop, fix it and fix other problems
      in following paragrah
   5. In Patch 07 use "drop out of RCU-walk"
   6. In Patch 08 "latter" should be "later", fix it and restructure the next paragrah
      removing "Finally"
v3:
 - Fix problems in v2 according to Neil's review:
   1. Fix minor problems in Patch 1,2,3,8,9,11,12
   2. In Patch 4 remove redundant paragraph, condense information, and
make the paragraph connection more smooth.
   3. In Patch 10 Fix WALK_NOFOLLOW, WALK_MORE descriptions and
document WALK_TRAILING

 - As suggested by Matthew Wilcox and Jonathan Corbet, remove ``...`` literals of function names in this patchset. I put this in a standalone Patch(Patch 13), because automarkup extension doesn't work on my side. You can choose to take it or not take it.


To help review, I've put a compiled html version here:
http://linux-docs.54fox.com/linux_docs/filesystems/path-lookup-v3.html
and highlight changes using hypothesis:
https://hyp.is/go?url=http%3A%2F%2Flinux-docs.54fox.com%2Flinux_docs%2Ffilesystems%2Fpath-lookup-v3.html&group=__world__


Fox Chen (13):
  docs: path-lookup: update follow_managed() part
  docs: path-lookup: update path_to_nameidata() part
  docs: path-lookup: update path_mountpoint() part
  docs: path-lookup: update do_last() part
  docs: path-lookup: remove filename_mountpoint
  docs: path-lookup: Add macro name to symlink limit description
  docs: path-lookup: i_op->follow_link replaced with i_op->get_link
  docs: path-lookup: update i_op->put_link and cookie description
  docs: path-lookup: no get_link()
  docs: path-lookup: update WALK_GET, WALK_PUT desc
  docs: path-lookup: update get_link() ->follow_link description
  docs: path-lookup: update symlink description
  docs: path-lookup: use bare function() rather than literals

 Documentation/filesystems/path-lookup.rst | 194 ++++++++++------------
 1 file changed, 85 insertions(+), 109 deletions(-)

-- 
2.31.1

[PATCH v3 01/13] docs: path-lookup: update follow_managed() part

[Fox Chen]

No follow_managed() anymore, handle_mounts(),
traverse_mounts(), will do the job.
see commit 9deed3ebca24 ("new helper: traverse_mounts()")

Signed-off-by: Fox Chen <foxhlchen@gmail.com>
---
 Documentation/filesystems/path-lookup.rst | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/Documentation/filesystems/path-lookup.rst b/Documentation/filesystems/path-lookup.rst
index c482e1619e77..751082d469e8 100644
--- a/Documentation/filesystems/path-lookup.rst
+++ b/Documentation/filesystems/path-lookup.rst
@@ -448,10 +448,11 @@ described.  If it finds a ``LAST_NORM`` component it first calls
 filesystem to revalidate the result if it is that sort of filesystem.
 If that doesn't get a good result, it calls "``lookup_slow()``" which
 takes ``i_rwsem``, rechecks the cache, and then asks the filesystem
-to find a definitive answer.  Each of these will call
-``follow_managed()`` (as described below) to handle any mount points.
+to find a definitive answer.
 
-In the absence of symbolic links, ``walk_component()`` creates a new
+As the last step of ``walk_component()``, ``step_into()`` will be called either
+directly from walk_component() or from handle_dots().  It calls
+``handle_mounts()``, to check and handle mount points, in which a new
 ``struct path`` containing a counted reference to the new dentry and a
 reference to the new ``vfsmount`` which is only counted if it is
 different from the previous ``vfsmount``.  It then calls
@@ -535,8 +536,7 @@ covered in greater detail in autofs.txt in the Linux documentation
 tree, but a few notes specifically related to path lookup are in order
 here.
 
-The Linux VFS has a concept of "managed" dentries which is reflected
-in function names such as "``follow_managed()``".  There are three
+The Linux VFS has a concept of "managed" dentries.  There are three
 potentially interesting things about these dentries corresponding
 to three different flags that might be set in ``dentry->d_flags``:
 
-- 
2.31.1

[PATCH v3 02/13] docs: path-lookup: update path_to_nameidata() part

[Fox Chen]

No path_to_namei() anymore, step_into() will be called.
Related commit: commit c99687a03a78 ("fold path_to_nameidata()
into its only remaining caller")

Signed-off-by: Fox Chen <foxhlchen@gmail.com>
---
 Documentation/filesystems/path-lookup.rst | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/Documentation/filesystems/path-lookup.rst b/Documentation/filesystems/path-lookup.rst
index 751082d469e8..6ea0880fb982 100644
--- a/Documentation/filesystems/path-lookup.rst
+++ b/Documentation/filesystems/path-lookup.rst
@@ -453,11 +453,12 @@ to find a definitive answer.
 As the last step of ``walk_component()``, ``step_into()`` will be called either
 directly from walk_component() or from handle_dots().  It calls
 ``handle_mounts()``, to check and handle mount points, in which a new
-``struct path`` containing a counted reference to the new dentry and a
-reference to the new ``vfsmount`` which is only counted if it is
-different from the previous ``vfsmount``.  It then calls
-``path_to_nameidata()`` to install the new ``struct path`` in the
-``struct nameidata`` and drop the unneeded references.
+``struct path`` is created containing a counted reference to the new dentry and
+a reference to the new ``vfsmount`` which is only counted if it is
+different from the previous ``vfsmount``. Then if there is
+a symbolic link, ``step_into()`` calls ``pick_link()`` to deal with it,
+otherwise it installs the new ``struct path`` in the ``struct nameidata``, and
+drops the unneeded references.
 
 This "hand-over-hand" sequencing of getting a reference to the new
 dentry before dropping the reference to the previous dentry may
-- 
2.31.1

[PATCH v3 03/13] docs: path-lookup: update path_mountpoint() part

[Fox Chen]

path_mountpoint() doesn't exist anymore. Have been folded
into path_lookup_at when flag is set with LOOKUP_MOUNTPOINT.
Check commit: commit 161aff1d93abf0e ("LOOKUP_MOUNTPOINT: fold
path_mountpointat() into path_lookupat()")

Signed-off-by: Fox Chen <foxhlchen@gmail.com>
---
 Documentation/filesystems/path-lookup.rst | 12 +++++-------
 1 file changed, 5 insertions(+), 7 deletions(-)

diff --git a/Documentation/filesystems/path-lookup.rst b/Documentation/filesystems/path-lookup.rst
index 6ea0880fb982..652d3284f178 100644
--- a/Documentation/filesystems/path-lookup.rst
+++ b/Documentation/filesystems/path-lookup.rst
@@ -472,7 +472,7 @@ Handling the final component
 ``nd->last_type`` to refer to the final component of the path.  It does
 not call ``walk_component()`` that last time.  Handling that final
 component remains for the caller to sort out. Those callers are
-``path_lookupat()``, ``path_parentat()``, ``path_mountpoint()`` and
+``path_lookupat()``, ``path_parentat()`` and
 ``path_openat()`` each of which handles the differing requirements of
 different system calls.
 
@@ -488,12 +488,10 @@ perform their operation.
 object is wanted such as by ``stat()`` or ``chmod()``.  It essentially just
 calls ``walk_component()`` on the final component through a call to
 ``lookup_last()``.  ``path_lookupat()`` returns just the final dentry.
-
-``path_mountpoint()`` handles the special case of unmounting which must
-not try to revalidate the mounted filesystem.  It effectively
-contains, through a call to ``mountpoint_last()``, an alternate
-implementation of ``lookup_slow()`` which skips that step.  This is
-important when unmounting a filesystem that is inaccessible, such as
+It is worth noting that when flag ``LOOKUP_MOUNTPOINT`` is set,
+``path_lookupat()`` will unset LOOKUP_JUMPED in nameidata so that in the
+subsequent path traversal ``d_weak_revalidate()`` won't be called.
+This is important when unmounting a filesystem that is inaccessible, such as
 one provided by a dead NFS server.
 
 Finally ``path_openat()`` is used for the ``open()`` system call; it
-- 
2.31.1

[PATCH v3 04/13] docs: path-lookup: update do_last() part

[Fox Chen]

traling_symlink() was merged into lookup_last, do_last().

do_last() has later been split into open_last_lookups()
and do_open().

see related commit: commit c5971b8c6354 ("take post-lookup
part of do_last() out of loop")

Signed-off-by: Fox Chen <foxhlchen@gmail.com>
---
 Documentation/filesystems/path-lookup.rst | 47 +++++++++++------------
 1 file changed, 22 insertions(+), 25 deletions(-)

diff --git a/Documentation/filesystems/path-lookup.rst b/Documentation/filesystems/path-lookup.rst
index 652d3284f178..2b0b33168067 100644
--- a/Documentation/filesystems/path-lookup.rst
+++ b/Documentation/filesystems/path-lookup.rst
@@ -495,11 +495,11 @@ This is important when unmounting a filesystem that is inaccessible, such as
 one provided by a dead NFS server.
 
 Finally ``path_openat()`` is used for the ``open()`` system call; it
-contains, in support functions starting with "``do_last()``", all the
+contains, in support functions starting with "``open_last_lookups()``", all the
 complexity needed to handle the different subtleties of O_CREAT (with
 or without O_EXCL), final "``/``" characters, and trailing symbolic
 links.  We will revisit this in the final part of this series, which
-focuses on those symbolic links.  "``do_last()``" will sometimes, but
+focuses on those symbolic links.  "``open_last_lookups()``" will sometimes, but
 not always, take ``i_rwsem``, depending on what it finds.
 
 Each of these, or the functions which call them, need to be alert to
@@ -1196,29 +1196,26 @@ potentially need to call ``link_path_walk()`` again and again on
 successive symlinks until one is found that doesn't point to another
 symlink.
 
-This case is handled by the relevant caller of ``link_path_walk()``, such as
-``path_lookupat()`` using a loop that calls ``link_path_walk()``, and then
-handles the final component.  If the final component is a symlink
-that needs to be followed, then ``trailing_symlink()`` is called to set
-things up properly and the loop repeats, calling ``link_path_walk()``
-again.  This could loop as many as 40 times if the last component of
-each symlink is another symlink.
-
-The various functions that examine the final component and possibly
-report that it is a symlink are ``lookup_last()``, ``mountpoint_last()``
-and ``do_last()``, each of which use the same convention as
-``walk_component()`` of returning ``1`` if a symlink was found that needs
-to be followed.
-
-Of these, ``do_last()`` is the most interesting as it is used for
-opening a file.  Part of ``do_last()`` runs with ``i_rwsem`` held and this
-part is in a separate function: ``lookup_open()``.
-
-Explaining ``do_last()`` completely is beyond the scope of this article,
-but a few highlights should help those interested in exploring the
-code.
-
-1. Rather than just finding the target file, ``do_last()`` needs to open
+This case is handled by relevant callers of ``link_path_walk()``, such as
+``path_lookupat()``, ``path_openat()`` using a loop that calls ``link_path_walk()``,
+and then handles the final component by calling ``open_last_lookups()`` or
+``lookup_last()``. If it is a symlink that needs to be followed,
+``open_last_lookups()`` or ``lookup_last()`` will set things up properly and
+return the path so that the loop repeats, calling
+``link_path_walk()`` again.  This could loop as many as 40 times if the last
+component of each symlink is another symlink.
+
+Of the various functions that examine the final component, 
+``open_last_lookups()`` is the most interesting as it works in tandem
+with ``do_open()`` for opening a file.  Part of ``open_last_lookups()`` runs
+with ``i_rwsem`` held and this part is in a separate function: ``lookup_open()``.
+
+Explaining ``open_last_lookups()`` and ``do_open()`` completely is beyond the scope
+of this article, but a few highlights should help those interested in exploring
+the code.
+
+1. Rather than just finding the target file, ``do_open()`` is used after
+   ``open_last_lookup()`` to open
    it.  If the file was found in the dcache, then ``vfs_open()`` is used for
    this.  If not, then ``lookup_open()`` will either call ``atomic_open()`` (if
    the filesystem provides it) to combine the final lookup with the open, or
-- 
2.31.1

[PATCH v3 05/13] docs: path-lookup: remove filename_mountpoint

[Fox Chen]

No filename_mountpoint any more
see commit: commit 161aff1d93ab ("LOOKUP_MOUNTPOINT:
fold path_mountpointat() into path_lookupat()")

Without filename_mountpoint and path_mountpoint(), the
numbers should be four & three:

"These four correspond roughly to the three path_*() functions"

Signed-off-by: Fox Chen <foxhlchen@gmail.com>
---
 Documentation/filesystems/path-lookup.rst | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/Documentation/filesystems/path-lookup.rst b/Documentation/filesystems/path-lookup.rst
index 2b0b33168067..3cbaf30b0f83 100644
--- a/Documentation/filesystems/path-lookup.rst
+++ b/Documentation/filesystems/path-lookup.rst
@@ -652,9 +652,9 @@ restarts from the top with REF-walk.
 
 This pattern of "try RCU-walk, if that fails try REF-walk" can be
 clearly seen in functions like ``filename_lookup()``,
-``filename_parentat()``, ``filename_mountpoint()``,
-``do_filp_open()``, and ``do_file_open_root()``.  These five
-correspond roughly to the four ``path_*()`` functions we met earlier,
+``filename_parentat()``,
+``do_filp_open()``, and ``do_file_open_root()``.  These four
+correspond roughly to the three ``path_*()`` functions we met earlier,
 each of which calls ``link_path_walk()``.  The ``path_*()`` functions are
 called using different mode flags until a mode is found which works.
 They are first called with ``LOOKUP_RCU`` set to request "RCU-walk".  If
-- 
2.31.1

[PATCH v3 06/13] docs: path-lookup: Add macro name to symlink limit description

[Fox Chen]

Add macro name MAXSYMLINKS to the symlink limit description, so
that it is consistent with path name length description above.

Signed-off-by: Fox Chen <foxhlchen@gmail.com>
---
 Documentation/filesystems/path-lookup.rst | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/Documentation/filesystems/path-lookup.rst b/Documentation/filesystems/path-lookup.rst
index 3cbaf30b0f83..40b9afec4d60 100644
--- a/Documentation/filesystems/path-lookup.rst
+++ b/Documentation/filesystems/path-lookup.rst
@@ -992,8 +992,8 @@ is 4096.  There are a number of reasons for this limit; not letting the
 kernel spend too much time on just one path is one of them.  With
 symbolic links you can effectively generate much longer paths so some
 sort of limit is needed for the same reason.  Linux imposes a limit of
-at most 40 symlinks in any one path lookup.  It previously imposed a
-further limit of eight on the maximum depth of recursion, but that was
+at most 40 (MAXSYMLINKS) symlinks in any one path lookup.  It previously imposed
+a further limit of eight on the maximum depth of recursion, but that was
 raised to 40 when a separate stack was implemented, so there is now
 just the one limit.
 
-- 
2.31.1

[PATCH v3 07/13] docs: path-lookup: i_op->follow_link replaced with i_op->get_link

[Fox Chen]

follow_link has been replaced by get_link() which can be
called in RCU mode.

see commit: commit 6b2553918d8b ("replace ->follow_link() with
new method that could stay in RCU mode")

Signed-off-by: Fox Chen <foxhlchen@gmail.com>
---
 Documentation/filesystems/path-lookup.rst | 12 +++++-------
 1 file changed, 5 insertions(+), 7 deletions(-)

diff --git a/Documentation/filesystems/path-lookup.rst b/Documentation/filesystems/path-lookup.rst
index 40b9afec4d60..4650c6427963 100644
--- a/Documentation/filesystems/path-lookup.rst
+++ b/Documentation/filesystems/path-lookup.rst
@@ -1060,13 +1060,11 @@ filesystem cannot successfully get a reference in RCU-walk mode, it
 must return ``-ECHILD`` and ``unlazy_walk()`` will be called to return to
 REF-walk mode in which the filesystem is allowed to sleep.
 
-The place for all this to happen is the ``i_op->follow_link()`` inode
-method.  In the present mainline code this is never actually called in
-RCU-walk mode as the rewrite is not quite complete.  It is likely that
-in a future release this method will be passed an ``inode`` pointer when
-called in RCU-walk mode so it both (1) knows to be careful, and (2) has the
-validated pointer.  Much like the ``i_op->permission()`` method we
-looked at previously, ``->follow_link()`` would need to be careful that
+The place for all this to happen is the ``i_op->get_link()`` inode
+method. This is called both in RCU-walk and REF-walk. In RCU-walk the
+``dentry*`` argument is NULL, ``->get_link()`` can return -ECHILD to drop out of
+RCU-walk.  Much like the ``i_op->permission()`` method we
+looked at previously, ``->get_link()`` would need to be careful that
 all the data structures it references are safe to be accessed while
 holding no counted reference, only the RCU lock.  Though getting a
 reference with ``->follow_link()`` is not yet done in RCU-walk mode, the
-- 
2.31.1

[PATCH v3 08/13] docs: path-lookup: update i_op->put_link and cookie description

[Fox Chen]

No inode->put_link operation anymore. We use delayed_call to
deal with link destruction. Cookie has been replaced with
struct delayed_call.

Related commit: commit fceef393a538 ("switch ->get_link() to
delayed_call, kill ->put_link()")

Signed-off-by: Fox Chen <foxhlchen@gmail.com>
---
 Documentation/filesystems/path-lookup.rst | 30 ++++++-----------------
 1 file changed, 8 insertions(+), 22 deletions(-)

diff --git a/Documentation/filesystems/path-lookup.rst b/Documentation/filesystems/path-lookup.rst
index 4650c6427963..3855809784cf 100644
--- a/Documentation/filesystems/path-lookup.rst
+++ b/Documentation/filesystems/path-lookup.rst
@@ -1066,34 +1066,20 @@ method. This is called both in RCU-walk and REF-walk. In RCU-walk the
 RCU-walk.  Much like the ``i_op->permission()`` method we
 looked at previously, ``->get_link()`` would need to be careful that
 all the data structures it references are safe to be accessed while
-holding no counted reference, only the RCU lock.  Though getting a
-reference with ``->follow_link()`` is not yet done in RCU-walk mode, the
-code is ready to release the reference when that does happen.
-
-This need to drop the reference to a symlink adds significant
-complexity.  It requires a reference to the inode so that the
-``i_op->put_link()`` inode operation can be called.  In REF-walk, that
-reference is kept implicitly through a reference to the dentry, so
-keeping the ``struct path`` of the symlink is easiest.  For RCU-walk,
-the pointer to the inode is kept separately.  To allow switching from
-RCU-walk back to REF-walk in the middle of processing nested symlinks
-we also need the seq number for the dentry so we can confirm that
-switching back was safe.
-
-Finally, when providing a reference to a symlink, the filesystem also
-provides an opaque "cookie" that must be passed to ``->put_link()`` so that it
-knows what to free.  This might be the allocated memory area, or a
-pointer to the ``struct page`` in the page cache, or something else
-completely.  Only the filesystem knows what it is.
+holding no counted reference, only the RCU lock. A callback
+``struct delayed_called`` will be passed to ``->get_link()``:
+file systems can set their own put_link function and argument through
+``set_delayed_call()``. Later on, when VFS wants to put link, it will call
+``do_delayed_call()`` to invoke that callback function with the argument.
 
 In order for the reference to each symlink to be dropped when the walk completes,
 whether in RCU-walk or REF-walk, the symlink stack needs to contain,
 along with the path remnants:
 
-- the ``struct path`` to provide a reference to the inode in REF-walk
-- the ``struct inode *`` to provide a reference to the inode in RCU-walk
+- the ``struct path`` to provide a reference to the previous path
+- the ``const char *`` to provide a reference to the to previous name
 - the ``seq`` to allow the path to be safely switched from RCU-walk to REF-walk
-- the ``cookie`` that tells ``->put_path()`` what to put.
+- the ``struct delayed_call`` for later invocation.
 
 This means that each entry in the symlink stack needs to hold five
 pointers and an integer instead of just one pointer (the path
-- 
2.31.1

[PATCH v3 09/13] docs: path-lookup: no get_link()

[Fox Chen]

no get_link() anymore. we have step_into() and pick_link().

walk_component() will call step_into(), in turn call pick_link,
and return symlink name.

Signed-off-by: Fox Chen <foxhlchen@gmail.com>
---
 Documentation/filesystems/path-lookup.rst | 10 ++++------
 1 file changed, 4 insertions(+), 6 deletions(-)

diff --git a/Documentation/filesystems/path-lookup.rst b/Documentation/filesystems/path-lookup.rst
index 3855809784cf..0a125673a8fe 100644
--- a/Documentation/filesystems/path-lookup.rst
+++ b/Documentation/filesystems/path-lookup.rst
@@ -1103,12 +1103,10 @@ doesn't need to notice.  Getting this ``name`` variable on and off the
 stack is very straightforward; pushing and popping the references is
 a little more complex.
 
-When a symlink is found, ``walk_component()`` returns the value ``1``
-(``0`` is returned for any other sort of success, and a negative number
-is, as usual, an error indicator).  This causes ``get_link()`` to be
-called; it then gets the link from the filesystem.  Providing that
-operation is successful, the old path ``name`` is placed on the stack,
-and the new value is used as the ``name`` for a while.  When the end of
+When a symlink is found, ``walk_component()`` calls ``pick_link()`` via ``step_into()``
+which returns the link from the filesystem.
+Providing that operation is successful, the old path ``name`` is placed on the
+stack, and the new value is used as the ``name`` for a while.  When the end of
 the path is found (i.e. ``*name`` is ``'\0'``) the old ``name`` is restored
 off the stack and path walking continues.
 
-- 
2.31.1

[PATCH v3 10/13] docs: path-lookup: update WALK_GET, WALK_PUT desc

[Fox Chen]

WALK_GET is changed to WALK_TRAILING with a different meaning.
Here it should be WALK_NOFOLLOW. WALK_PUT dosn't exist, we have
WALK_MORE.

WALK_PUT == !WALK_MORE

And there is not should_follow_link().

Related commits:
commit 8c4efe22e7c4 ("namei: invert the meaning of WALK_FOLLOW")
commit 1c4ff1a87e46 ("namei: invert WALK_PUT logics")

Signed-off-by: Fox Chen <foxhlchen@gmail.com>
---
 Documentation/filesystems/path-lookup.rst | 14 +++++++-------
 1 file changed, 7 insertions(+), 7 deletions(-)

diff --git a/Documentation/filesystems/path-lookup.rst b/Documentation/filesystems/path-lookup.rst
index 0a125673a8fe..08e6306af5b1 100644
--- a/Documentation/filesystems/path-lookup.rst
+++ b/Documentation/filesystems/path-lookup.rst
@@ -1123,13 +1123,13 @@ stack in ``walk_component()`` immediately when the symlink is found;
 old symlink as it walks that last component.  So it is quite
 convenient for ``walk_component()`` to release the old symlink and pop
 the references just before pushing the reference information for the
-new symlink.  It is guided in this by two flags; ``WALK_GET``, which
-gives it permission to follow a symlink if it finds one, and
-``WALK_PUT``, which tells it to release the current symlink after it has been
-followed.  ``WALK_PUT`` is tested first, leading to a call to
-``put_link()``.  ``WALK_GET`` is tested subsequently (by
-``should_follow_link()``) leading to a call to ``pick_link()`` which sets
-up the stack frame.
+new symlink.  It is guided in this by three flags: ``WALK_NOFOLLOW`` which
+forbits it from following a symlink if it finds one, ``WALK_MORE``
+which indicates that it is yet too early to release the
+current symlink, and ``WALK_TRAILING`` which predicates that it is on the final
+component of the lookup, so we will check userspace flag ``LOOKUP_FOLLOW`` to
+decide whether follow it when it is a symlink and call ``may_follow_link()`` to
+check if we have privilege to follow it.
 
 Symlinks with no final component
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-- 
2.31.1

Re: [PATCH v3 10/13] docs: path-lookup: update WALK_GET, WALK_PUT desc

[NeilBrown]

On Thu, 27 May 2021, Fox Chen wrote:
> WALK_GET is changed to WALK_TRAILING with a different meaning.
> Here it should be WALK_NOFOLLOW. WALK_PUT dosn't exist, we have
> WALK_MORE.
> 
> WALK_PUT == !WALK_MORE
> 
> And there is not should_follow_link().
> 
> Related commits:
> commit 8c4efe22e7c4 ("namei: invert the meaning of WALK_FOLLOW")
> commit 1c4ff1a87e46 ("namei: invert WALK_PUT logics")
> 
> Signed-off-by: Fox Chen <foxhlchen@gmail.com>
> ---
>  Documentation/filesystems/path-lookup.rst | 14 +++++++-------
>  1 file changed, 7 insertions(+), 7 deletions(-)
> 
> diff --git a/Documentation/filesystems/path-lookup.rst b/Documentation/filesystems/path-lookup.rst
> index 0a125673a8fe..08e6306af5b1 100644
> --- a/Documentation/filesystems/path-lookup.rst
> +++ b/Documentation/filesystems/path-lookup.rst
> @@ -1123,13 +1123,13 @@ stack in ``walk_component()`` immediately when the symlink is found;
>  old symlink as it walks that last component.  So it is quite
>  convenient for ``walk_component()`` to release the old symlink and pop
>  the references just before pushing the reference information for the
> -new symlink.  It is guided in this by two flags; ``WALK_GET``, which
> -gives it permission to follow a symlink if it finds one, and
> -``WALK_PUT``, which tells it to release the current symlink after it has been
> -followed.  ``WALK_PUT`` is tested first, leading to a call to
> -``put_link()``.  ``WALK_GET`` is tested subsequently (by
> -``should_follow_link()``) leading to a call to ``pick_link()`` which sets
> -up the stack frame.
> +new symlink.  It is guided in this by three flags: ``WALK_NOFOLLOW`` which
> +forbits it from following a symlink if it finds one, ``WALK_MORE``

"forbids"

> +which indicates that it is yet too early to release the
> +current symlink, and ``WALK_TRAILING`` which predicates that it is on the final

"predicates" isn't quite the right word to use here.  "indicates" is
better, but we've already used that word in the sentence, and repetition
feels clumsy (I wonder if anyone else listened to "My Word" on the BBC -
no hesitation, repetition, or deviation). 
Maybe "reports"?  Or maybe out chief editor can make a suggestion.

NeilBrown

> +component of the lookup, so we will check userspace flag ``LOOKUP_FOLLOW`` to
> +decide whether follow it when it is a symlink and call ``may_follow_link()`` to
> +check if we have privilege to follow it.
>  
>  Symlinks with no final component
>  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> -- 
> 2.31.1
> 
> 

[PATCH v3 11/13] docs: path-lookup: update get_link() ->follow_link description

[Fox Chen]

get_link() is merged into pick_link(). i_op->follow_link is
replaced with i_op->get_link(). get_link() can return ERR_PTR(0)
which equals NULL.

Signed-off-by: Fox Chen <foxhlchen@gmail.com>
---
 Documentation/filesystems/path-lookup.rst | 13 ++++++-------
 1 file changed, 6 insertions(+), 7 deletions(-)

diff --git a/Documentation/filesystems/path-lookup.rst b/Documentation/filesystems/path-lookup.rst
index 08e6306af5b1..3f03fbbc9429 100644
--- a/Documentation/filesystems/path-lookup.rst
+++ b/Documentation/filesystems/path-lookup.rst
@@ -1136,10 +1136,10 @@ Symlinks with no final component
 
 A pair of special-case symlinks deserve a little further explanation.
 Both result in a new ``struct path`` (with mount and dentry) being set
-up in the ``nameidata``, and result in ``get_link()`` returning ``NULL``.
+up in the ``nameidata``, and result in ``pick_link()`` returning ``NULL``.
 
 The more obvious case is a symlink to "``/``".  All symlinks starting
-with "``/``" are detected in ``get_link()`` which resets the ``nameidata``
+with "``/``" are detected in ``pick_link()`` which resets the ``nameidata``
 to point to the effective filesystem root.  If the symlink only
 contains "``/``" then there is nothing more to do, no components at all,
 so ``NULL`` is returned to indicate that the symlink can be released and
@@ -1156,12 +1156,11 @@ something that looks like a symlink.  It is really a reference to the
 target file, not just the name of it.  When you ``readlink`` these
 objects you get a name that might refer to the same file - unless it
 has been unlinked or mounted over.  When ``walk_component()`` follows
-one of these, the ``->follow_link()`` method in "procfs" doesn't return
+one of these, the ``->get_link()`` method in "procfs" doesn't return
 a string name, but instead calls ``nd_jump_link()`` which updates the
-``nameidata`` in place to point to that target.  ``->follow_link()`` then
-returns ``NULL``.  Again there is no final component and ``get_link()``
-reports this by leaving the ``last_type`` field of ``nameidata`` as
-``LAST_BIND``.
+``nameidata`` in place to point to that target.  ``->get_link()`` then
+returns ``NULL``.  Again there is no final component and ``pick_link()``
+returns ``NULL``.
 
 Following the symlink in the final component
 --------------------------------------------
-- 
2.31.1

[PATCH v3 12/13] docs: path-lookup: update symlink description

[Fox Chen]

instead of lookup_real()/vfs_create(), i_op->lookup() and
i_op->create() will be called directly.

update vfs_open() logic

should_follow_link is merged into lookup_last() or open_last_lookup()
which returns symlink name instead of an integer.

Signed-off-by: Fox Chen <foxhlchen@gmail.com>
---
 Documentation/filesystems/path-lookup.rst | 13 ++++++-------
 1 file changed, 6 insertions(+), 7 deletions(-)

diff --git a/Documentation/filesystems/path-lookup.rst b/Documentation/filesystems/path-lookup.rst
index 3f03fbbc9429..9ac742530e46 100644
--- a/Documentation/filesystems/path-lookup.rst
+++ b/Documentation/filesystems/path-lookup.rst
@@ -1200,16 +1200,15 @@ the code.
    it.  If the file was found in the dcache, then ``vfs_open()`` is used for
    this.  If not, then ``lookup_open()`` will either call ``atomic_open()`` (if
    the filesystem provides it) to combine the final lookup with the open, or
-   will perform the separate ``lookup_real()`` and ``vfs_create()`` steps
+   will perform the separate ``i_op->lookup()`` and ``i_op->create()`` steps
    directly.  In the later case the actual "open" of this newly found or
    created file will be performed by ``vfs_open()``, just as if the name
    were found in the dcache.
 
 2. ``vfs_open()`` can fail with ``-EOPENSTALE`` if the cached information
-   wasn't quite current enough.  Rather than restarting the lookup from
-   the top with ``LOOKUP_REVAL`` set, ``lookup_open()`` is called instead,
-   giving the filesystem a chance to resolve small inconsistencies.
-   If that doesn't work, only then is the lookup restarted from the top.
+   wasn't quite current enough.  If it's in RCU-walk ``-ECHILD`` will be returned
+   otherwise ``-ESTALE`` is returned.  When ``-ESTALE`` is returned, the caller may
+   retry with ``LOOKUP_REVAL`` flag set.
 
 3. An open with O_CREAT **does** follow a symlink in the final component,
    unlike other creation system calls (like ``mkdir``).  So the sequence::
@@ -1219,8 +1218,8 @@ the code.
 
    will create a file called ``/tmp/bar``.  This is not permitted if
    ``O_EXCL`` is set but otherwise is handled for an O_CREAT open much
-   like for a non-creating open: ``should_follow_link()`` returns ``1``, and
-   so does ``do_last()`` so that ``trailing_symlink()`` gets called and the
+   like for a non-creating open: ``lookup_last()`` or ``open_last_lookup()``
+   returns a non ``NULL`` value, and ``link_path_walk()`` gets called and the
    open process continues on the symlink that was found.
 
 Updating the access time
-- 
2.31.1

[PATCH v3 13/13] docs: path-lookup: use bare function() rather than literals

[Fox Chen]

As suggested by Matthew Wilcox and Jonathan Corbet, drop ``...``
literals around function names of this patchset.

Signed-off-by: Fox Chen <foxhlchen@gmail.com>
---
 Documentation/filesystems/path-lookup.rst | 70 +++++++++++------------
 1 file changed, 35 insertions(+), 35 deletions(-)

diff --git a/Documentation/filesystems/path-lookup.rst b/Documentation/filesystems/path-lookup.rst
index 9ac742530e46..33d58fca662a 100644
--- a/Documentation/filesystems/path-lookup.rst
+++ b/Documentation/filesystems/path-lookup.rst
@@ -450,13 +450,13 @@ If that doesn't get a good result, it calls "``lookup_slow()``" which
 takes ``i_rwsem``, rechecks the cache, and then asks the filesystem
 to find a definitive answer.
 
-As the last step of ``walk_component()``, ``step_into()`` will be called either
+As the last step of walk_component(), step_into() will be called either
 directly from walk_component() or from handle_dots().  It calls
-``handle_mounts()``, to check and handle mount points, in which a new
+handle_mounts(), to check and handle mount points, in which a new
 ``struct path`` is created containing a counted reference to the new dentry and
 a reference to the new ``vfsmount`` which is only counted if it is
 different from the previous ``vfsmount``. Then if there is
-a symbolic link, ``step_into()`` calls ``pick_link()`` to deal with it,
+a symbolic link, step_into() calls pick_link() to deal with it,
 otherwise it installs the new ``struct path`` in the ``struct nameidata``, and
 drops the unneeded references.
 
@@ -472,8 +472,8 @@ Handling the final component
 ``nd->last_type`` to refer to the final component of the path.  It does
 not call ``walk_component()`` that last time.  Handling that final
 component remains for the caller to sort out. Those callers are
-``path_lookupat()``, ``path_parentat()`` and
-``path_openat()`` each of which handles the differing requirements of
+path_lookupat(), path_parentat() and
+path_openat() each of which handles the differing requirements of
 different system calls.
 
 ``path_parentat()`` is clearly the simplest - it just wraps a little bit
@@ -489,17 +489,17 @@ object is wanted such as by ``stat()`` or ``chmod()``.  It essentially just
 calls ``walk_component()`` on the final component through a call to
 ``lookup_last()``.  ``path_lookupat()`` returns just the final dentry.
 It is worth noting that when flag ``LOOKUP_MOUNTPOINT`` is set,
-``path_lookupat()`` will unset LOOKUP_JUMPED in nameidata so that in the
-subsequent path traversal ``d_weak_revalidate()`` won't be called.
+path_lookupat() will unset LOOKUP_JUMPED in nameidata so that in the
+subsequent path traversal d_weak_revalidate() won't be called.
 This is important when unmounting a filesystem that is inaccessible, such as
 one provided by a dead NFS server.
 
 Finally ``path_openat()`` is used for the ``open()`` system call; it
-contains, in support functions starting with "``open_last_lookups()``", all the
+contains, in support functions starting with "open_last_lookups()", all the
 complexity needed to handle the different subtleties of O_CREAT (with
 or without O_EXCL), final "``/``" characters, and trailing symbolic
 links.  We will revisit this in the final part of this series, which
-focuses on those symbolic links.  "``open_last_lookups()``" will sometimes, but
+focuses on those symbolic links.  "open_last_lookups()" will sometimes, but
 not always, take ``i_rwsem``, depending on what it finds.
 
 Each of these, or the functions which call them, need to be alert to
@@ -651,9 +651,9 @@ RCU-walk finds it cannot stop gracefully, it simply gives up and
 restarts from the top with REF-walk.
 
 This pattern of "try RCU-walk, if that fails try REF-walk" can be
-clearly seen in functions like ``filename_lookup()``,
-``filename_parentat()``,
-``do_filp_open()``, and ``do_file_open_root()``.  These four
+clearly seen in functions like filename_lookup(),
+filename_parentat(),
+do_filp_open(), and do_file_open_root().  These four
 correspond roughly to the three ``path_*()`` functions we met earlier,
 each of which calls ``link_path_walk()``.  The ``path_*()`` functions are
 called using different mode flags until a mode is found which works.
@@ -1069,8 +1069,8 @@ all the data structures it references are safe to be accessed while
 holding no counted reference, only the RCU lock. A callback
 ``struct delayed_called`` will be passed to ``->get_link()``:
 file systems can set their own put_link function and argument through
-``set_delayed_call()``. Later on, when VFS wants to put link, it will call
-``do_delayed_call()`` to invoke that callback function with the argument.
+set_delayed_call(). Later on, when VFS wants to put link, it will call
+do_delayed_call() to invoke that callback function with the argument.
 
 In order for the reference to each symlink to be dropped when the walk completes,
 whether in RCU-walk or REF-walk, the symlink stack needs to contain,
@@ -1103,7 +1103,7 @@ doesn't need to notice.  Getting this ``name`` variable on and off the
 stack is very straightforward; pushing and popping the references is
 a little more complex.
 
-When a symlink is found, ``walk_component()`` calls ``pick_link()`` via ``step_into()``
+When a symlink is found, walk_component() calls pick_link() via step_into()
 which returns the link from the filesystem.
 Providing that operation is successful, the old path ``name`` is placed on the
 stack, and the new value is used as the ``name`` for a while.  When the end of
@@ -1136,10 +1136,10 @@ Symlinks with no final component
 
 A pair of special-case symlinks deserve a little further explanation.
 Both result in a new ``struct path`` (with mount and dentry) being set
-up in the ``nameidata``, and result in ``pick_link()`` returning ``NULL``.
+up in the ``nameidata``, and result in pick_link() returning ``NULL``.
 
 The more obvious case is a symlink to "``/``".  All symlinks starting
-with "``/``" are detected in ``pick_link()`` which resets the ``nameidata``
+with "``/``" are detected in pick_link() which resets the ``nameidata``
 to point to the effective filesystem root.  If the symlink only
 contains "``/``" then there is nothing more to do, no components at all,
 so ``NULL`` is returned to indicate that the symlink can be released and
@@ -1157,9 +1157,9 @@ target file, not just the name of it.  When you ``readlink`` these
 objects you get a name that might refer to the same file - unless it
 has been unlinked or mounted over.  When ``walk_component()`` follows
 one of these, the ``->get_link()`` method in "procfs" doesn't return
-a string name, but instead calls ``nd_jump_link()`` which updates the
+a string name, but instead calls nd_jump_link() which updates the
 ``nameidata`` in place to point to that target.  ``->get_link()`` then
-returns ``NULL``.  Again there is no final component and ``pick_link()``
+returns ``NULL``.  Again there is no final component and pick_link()
 returns ``NULL``.
 
 Following the symlink in the final component
@@ -1177,35 +1177,35 @@ potentially need to call ``link_path_walk()`` again and again on
 successive symlinks until one is found that doesn't point to another
 symlink.
 
-This case is handled by relevant callers of ``link_path_walk()``, such as
-``path_lookupat()``, ``path_openat()`` using a loop that calls ``link_path_walk()``,
-and then handles the final component by calling ``open_last_lookups()`` or
-``lookup_last()``. If it is a symlink that needs to be followed,
-``open_last_lookups()`` or ``lookup_last()`` will set things up properly and
+This case is handled by relevant callers of link_path_walk(), such as
+path_lookupat(), path_openat() using a loop that calls link_path_walk(),
+and then handles the final component by calling open_last_lookups() or
+lookup_last(). If it is a symlink that needs to be followed,
+open_last_lookups() or lookup_last() will set things up properly and
 return the path so that the loop repeats, calling
-``link_path_walk()`` again.  This could loop as many as 40 times if the last
+link_path_walk() again.  This could loop as many as 40 times if the last
 component of each symlink is another symlink.
 
 Of the various functions that examine the final component, 
-``open_last_lookups()`` is the most interesting as it works in tandem
-with ``do_open()`` for opening a file.  Part of ``open_last_lookups()`` runs
-with ``i_rwsem`` held and this part is in a separate function: ``lookup_open()``.
+open_last_lookups() is the most interesting as it works in tandem
+with do_open() for opening a file.  Part of open_last_lookups() runs
+with ``i_rwsem`` held and this part is in a separate function: lookup_open().
 
-Explaining ``open_last_lookups()`` and ``do_open()`` completely is beyond the scope
+Explaining open_last_lookups() and do_open() completely is beyond the scope
 of this article, but a few highlights should help those interested in exploring
 the code.
 
-1. Rather than just finding the target file, ``do_open()`` is used after
-   ``open_last_lookup()`` to open
+1. Rather than just finding the target file, do_open() is used after
+   open_last_lookup() to open
    it.  If the file was found in the dcache, then ``vfs_open()`` is used for
    this.  If not, then ``lookup_open()`` will either call ``atomic_open()`` (if
    the filesystem provides it) to combine the final lookup with the open, or
    will perform the separate ``i_op->lookup()`` and ``i_op->create()`` steps
    directly.  In the later case the actual "open" of this newly found or
-   created file will be performed by ``vfs_open()``, just as if the name
+   created file will be performed by vfs_open(), just as if the name
    were found in the dcache.
 
-2. ``vfs_open()`` can fail with ``-EOPENSTALE`` if the cached information
+2. vfs_open() can fail with ``-EOPENSTALE`` if the cached information
    wasn't quite current enough.  If it's in RCU-walk ``-ECHILD`` will be returned
    otherwise ``-ESTALE`` is returned.  When ``-ESTALE`` is returned, the caller may
    retry with ``LOOKUP_REVAL`` flag set.
@@ -1218,8 +1218,8 @@ the code.
 
    will create a file called ``/tmp/bar``.  This is not permitted if
    ``O_EXCL`` is set but otherwise is handled for an O_CREAT open much
-   like for a non-creating open: ``lookup_last()`` or ``open_last_lookup()``
-   returns a non ``NULL`` value, and ``link_path_walk()`` gets called and the
+   like for a non-creating open: lookup_last() or open_last_lookup()
+   returns a non ``NULL`` value, and link_path_walk() gets called and the
    open process continues on the symlink that was found.
 
 Updating the access time
-- 
2.31.1

Re: [PATCH v3 00/13] docs: path-lookup: Update pathlookup docs

[NeilBrown]

On Thu, 27 May 2021, Fox Chen wrote:
> The Path lookup is a very complex subject in VFS. The path-lookup
> document provides a very detailed guidance to help people understand
> how path lookup works in the kernel. This document was originally
> written based on three lwn articles five years ago. As times goes by,
> some of the content is outdated. This patchset is intended to update
> the document to make it more relevant to current codebase.
> 

Thanks for persisting.  Sorry for the delay.

All:
  Reviewed-by: NeilBrown <neilb@suse.de>

I've noted a couple of little issues with one patch.  Hopefully Jon can
simply fix those up rather than requiring a resubmission of the whole
series.

To be honest, I haven't examined patch 4 in as much detail as I'd like,
and it required the biggest change since last time.  But I think it is
good enough.  It might even be excellent.

NeilBrown

Re: [PATCH v3 00/13] docs: path-lookup: Update pathlookup docs

[Fox Chen]

On Fri, Jun 18, 2021 at 6:36 AM NeilBrown <neilb@suse.de> wrote:
>
> On Thu, 27 May 2021, Fox Chen wrote:
> > The Path lookup is a very complex subject in VFS. The path-lookup
> > document provides a very detailed guidance to help people understand
> > how path lookup works in the kernel. This document was originally
> > written based on three lwn articles five years ago. As times goes by,
> > some of the content is outdated. This patchset is intended to update
> > the document to make it more relevant to current codebase.
> >
>
> Thanks for persisting.  Sorry for the delay.

Thanks for the review. :D

> All:
>   Reviewed-by: NeilBrown <neilb@suse.de>
>
> I've noted a couple of little issues with one patch.  Hopefully Jon can
> simply fix those up rather than requiring a resubmission of the whole
> series.

if needed, I can resubmit just this single patch.

> To be honest, I haven't examined patch 4 in as much detail as I'd like,
> and it required the biggest change since last time.  But I think it is
> good enough.  It might even be excellent.
>
> NeilBrown

thanks,
fox

Re: [PATCH v3 00/13] docs: path-lookup: Update pathlookup docs

[Jonathan Corbet]

Fox Chen <foxhlchen@gmail.com> writes:

> The Path lookup is a very complex subject in VFS. The path-lookup
> document provides a very detailed guidance to help people understand
> how path lookup works in the kernel. This document was originally
> written based on three lwn articles five years ago. As times goes by,
> some of the content is outdated. This patchset is intended to update
> the document to make it more relevant to current codebase.

OK, I have applied this set.  I took the liberty of making the changes
suggested by Neil to patch 10.

Thanks for doing this work, and thanks to Neil for reviewing it!

jon