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From: Matthew Wilcox <>
To: David Howells <>
Cc: Alexander Viro <>,
	Miklos Szeredi <>, Ian Kent <>,
	Linus Torvalds <>,,
Subject: Re: [RFC][PATCH 0/3] vfs: Use an xarray instead of inserted bookmarks to scan mount list
Date: Mon, 15 Mar 2021 12:46:44 +0000	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

On Mon, Mar 15, 2021 at 12:07:39PM +0000, David Howells wrote:
> Hi Al, Miklós,
> Can we consider replacing the "insert cursor" approach we're currently
> using for proc files to scan the current namespace's mount list[1] with
> something that uses an xarray of mounts indexed by mnt_id?
> This has some advantages:
>  (1) It's simpler.  We don't need to insert dummy mount objects as
>      bookmarks into the mount list and code that's walking the list doesn't
>      have to carefully step over them.
>  (2) We can use the file position to represent the mnt_id and can jump to
>      it directly - ie. using seek() to jump to a mount object by its ID.
>  (3) It might make it easier to use RCU in future to dump mount entries
>      rather than having to take namespace_sem.  xarray provides for the
>      possibility of tagging entries to say that they're viewable to avoid
>      dumping incomplete mount objects.

Usually one fully constructs the object, then inserts it into the XArray.

> But there are a number of disadvantages:
>  (1) We have to allocate memory to maintain the xarray, which becomes more
>      of a problem as mnt_id values get scattered.

mnt_id values don't seem to get particularly scattered.  They're allocated
using an IDA, so they stay small (unlike someone using idr_alloc_cyclic

  parent reply	other threads:[~2021-03-15 12:48 UTC|newest]

Thread overview: 10+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-03-15 12:07 [RFC][PATCH 0/3] vfs: Use an xarray instead of inserted bookmarks to scan mount list David Howells
2021-03-15 12:07 ` [PATCH 1/3] vfs: Use an xarray in the mount namespace to handle /proc/mounts list David Howells
2021-03-15 12:07 ` [PATCH 2/3] vfs: Use the mounts_to_id array to do /proc/mounts and co David Howells
2021-03-15 12:54   ` Matthew Wilcox
2021-03-15 12:08 ` [PATCH 3/3] vfs: Remove mount list trawling cursor stuff David Howells
2021-03-15 12:46 ` Matthew Wilcox [this message]
2021-03-15 13:14 ` [RFC][PATCH 0/3] vfs: Use an xarray instead of inserted bookmarks to scan mount list Miklos Szeredi
2021-03-15 13:17   ` Matthew Wilcox
2021-03-15 13:41 ` David Howells
2021-03-15 14:22   ` Miklos Szeredi

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