Re: How to read-protect a vm_area?

Re: How to read-protect a vm_area?

[Benjamin C.R. LaHaise]

On Thu, 19 Feb 1998, Stephen C. Tweedie wrote:
...
> Please do let me know if you want to start hacking around with this
> code --- we probably want to coordinate with some of the other VM
> things happening at the moment (in particular, things like Ingo's swap
> prediction and the dirty page caching suggestions).
...

Just to let people know, as a successor to my pte-list/swapping patch from
the 2.1.48/66 days (which made running X on my nfs-root'd [34]86
possible/reliable), I'm currently mostly done a patch that does Mach-style
page replacement (active/inactive/free) as an alternative to kswapd.  I'm
also hoping to work on adding a per-cpu free page cache to get_free_page
later this month when my PPros arrive (grumble).

As Rik mentioned, please feel free to make use of linux-mm@kvack.org for
discussion purposes. (echo subscribe | mail majordomo@kvack.org)  It's
been quite, but then it's Febuary.

About the dirty page caching suggestions:  Eric W. Biederman
<ebiederm+eric@npwt.net> wrote patches to support that against 2.1.78, but
last time the issue was brought up, things became messy as NFS needs
dentries now, yet we'll only ever have inodes in struct page.  Now that
the dentry list is back in the inode, perhaps a patch to revert
read/writepage to non-dentry arguments could be accepted?  (NFS could get
its dentry from the i_dentry list.)

Linus: how far off are you hoping for 2.2?  It seems like there are
icebreakers out on the first code freeze...  Or maybe that just how things
work ;-)

		-ben

PATCH: Swap shared pages (was: How to read-protect a vm_area?)

[Stephen C. Tweedie]

Hi,

On Fri, 20 Feb 1998 00:41:19 -0500 (EST), "Benjamin C.R. LaHaise"
<blah@kvack.org> said:

> As Rik mentioned, please feel free to make use of linux-mm@kvack.org for
> discussion purposes. (echo subscribe | mail majordomo@kvack.org)  It's
> been quite, but then it's Febuary.

OK.  I'm CC:ing there.

The patch below, against 2.1.88, adds a bunch of new functionality to
the swapper.  The main changes are:

* All swapping goes through the swap cache (aka. page cache) now.

* There is no longer a swap lock map.  Because we need to atomically
  test and create a new swap-cache page in order to do swap IO, it is
  sufficient just to lock the struct page itself.  Having only one
  layer of locking to deal with removes a number of races concerning 
  swapping shared pages.

* We can swap shared pages, and still keep them shared when they are
  swapped back in!!!  Currently, only private shared pages (as in pages
  shared after a fork()) benefit from this, but the basic mechanism will
  be appropriate for MAP_ANONYMOUS | MAP_SHARED pages too
  (implementation to follow).  Pages will remain shared after a swapoff.

* The page cache is now quite happy dealing with swap-cache pages too.
  In particular, write-ahead and read-ahead of swap through the page
  cache will work fine (and in fact, write-ahead does get done already
  under certain circumstances with this patch --- that's essentially how
  the swapping of shared pages gets done).  Support code to perform
  asynchronous readahead of swap is included, but is not actually used
  anywhere yet.

I've tested with a number of forked processes running with a shared
working set larger than physical memory, and with SysV shared memory.
I haven't found any problems with it so far.

Enjoy.

Cheers,
 Stephen.

----------------------------------------------------------------
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end

Re: PATCH: Swap shared pages (was: How to read-protect a vm_area?)

[Linus Torvalds]

On Mon, 23 Feb 1998, Stephen C. Tweedie wrote:
> 
> The patch below, against 2.1.88, adds a bunch of new functionality to
> the swapper.  The main changes are:

Ok, this looks clean, I've applied it to my current sources and pending no
surprises it will be in 89. 

[ I've also changed the way we consider us to need more memory in kswapd,
  but that was entirely orthogonal and did not impact these patches. ]

Knock wood,

		Linus

Re: PATCH: Swap shared pages (was: How to read-protect a vm_area?)

[Benjamin C.R. LaHaise]

Hello,

On Mon, 23 Feb 1998, Stephen C. Tweedie wrote:
...
> The patch below, against 2.1.88, adds a bunch of new functionality to
> the swapper.  The main changes are:
> 
> * All swapping goes through the swap cache (aka. page cache) now.
...

I noticed you're using just one inode for the swapper/page cache...  What
I've been working on is a slightly different approach:  Create inodes for
each anonymous mapping.  The actual implementation uses one inode per
mm_struct, with the virtual address within the process providing the
offset.  This has the advantage of giving us an easy way to find all ptes
that use an anonymous page.  Anonymous mappings end up looking more like
shared mappings, which gives us some interesting possibilities - it
becomes almost trivial to implement a MAP_SHARED on another process'
address space.  What do you think of this approach?  My main goal is to
reimplement the page-oriented swapping my pte-list patch performed, which
makes the running time try_to_free_page drastically shorter, even
predictable... (at most 1 pass over mem_map to find a page using the old
style aging, or just one list operation using the inactive list approach) 

		-ben

Re: PATCH: Swap shared pages (was: How to read-protect a vm_area?)

[Stephen C. Tweedie]

Hi Ben,

On Mon, 23 Feb 1998 19:08:59 -0500 (U\x01), "Benjamin C.R. LaHaise"
<blah@kvack.org> said:

> Hello,
> On Mon, 23 Feb 1998, Stephen C. Tweedie wrote:
> ...
>> The patch below, against 2.1.88, adds a bunch of new functionality to
>> the swapper.  The main changes are:
>> 
>> * All swapping goes through the swap cache (aka. page cache) now.
> ...

> I noticed you're using just one inode for the swapper/page cache...  What
> I've been working on is a slightly different approach:  Create inodes for
> each anonymous mapping.  

It's not a different approach to the same problem --- it's a different
problem entirely!  The swapper_inode is *only* used as a root for the
page cache.  Its job is to identify pages by their swap entry, rather
than by their vma.  Its purpose is really more to do with the management
of swap pages on disk than in memory.

> The actual implementation uses one inode per mm_struct, with the
> virtual address within the process providing the offset.  This has the
> advantage of giving us an easy way to find all ptes that use an
> anonymous page.  Anonymous mappings end up looking more like shared
> mappings, which gives us some interesting possibilities - it becomes
> almost trivial to implement a MAP_SHARED on another process' address
> space.  What do you think of this approach?  

I'm not sure --- one inode per mm might have problems if we ever change
the virtual address of a physical page (and mremap() does exactly that).
However, that's not an insurmountable problem, and the remap-vma code
will probably get it right.  In fact, the more I think of it the more I
am convinced that this is a good way to go.

I am actually planning a different but very similar approach for the
final MAP_SHARED | MAP_ANONYMOUS code, which is to have one inode per
new vma for anonymous shared regions.  The primary reason for that is
for lookup, so that when we initialise a demand-zero page, we can
rapidly locate any other processes sharing this vma and update their
pte's too.

> My main goal is to reimplement the page-oriented swapping my pte-list
> patch performed, which makes the running time try_to_free_page
> drastically shorter, even predictable... (at most 1 pass over mem_map
> to find a page using the old style aging, or just one list operation
> using the inactive list approach)

Yep.  I was thinking along similar lines a while back.  Doing this will
also make it easier to unify the handling of shrink_mmap() and
try_to_free_page(), which is something we desparately need (we've
already unified the page and buffer shrinking, and I think we can unify
shm swapout too with the new swap cache code).

The changes you are proposing overlap a lot of my current patches, but
that's not a problem --- the two sets of changes doing fundamentally
orthogonal things; there's just an overlap in the middle.  The code I'm
working on right now is targetted at getting MAP_SHARED | MAP_ANONYMOUS
in place, and I reckon it's now pretty close.  

However, the new swap cache mechanism is a lot more generic than that,
and its real flexibility lies in the way its underlying mechanism works
--- the ability to do swap read-ahead and to proactively write-ahead
swap pages will allow us to do some major performance enhancements.
Your changes to the vmscan code are really concerned with policy ---
rapidly locating what to swap, where and when --- than the mechanics of
getting pages to and from disk, synchronously or asynchronously.  In
other words, I'm keen to integrate the two diffs, since I see a lot more
complimentary than overlapping progress here.

Cheers,
 Stephen.

Re: PATCH: Swap shared pages (was: How to read-protect a vm_area?)

[Rik van Riel]

[linux-kernel trimmed from f-ups]

On Mon, 23 Feb 1998, Stephen C. Tweedie wrote:

> The patch below, against 2.1.88, adds a bunch of new functionality to
> the swapper.  The main changes are:
> 
> * All swapping goes through the swap cache (aka. page cache) now.

Does this mean that _after_ the pages are properly aged
as user-pages, they'll be aged again as page-cache pages?
(when proper aging is added to the page cache, by eg. my patch)

I think it might be far better to:
- put user-pages in the swap cache after they haven't been used
  for two aging rounds
- free swap-cache pages and page-cache pages after they haven't
  been used for eight aging rounds (so the real aging and waiting
  takes place here)
- use right-shift aging here {age << 1; if(touched) age |= 0x80}
- adapt the get_free_pages so it can allocate clean page-cache and
  swap-cache pages when:
  - a bigorder area can't be found
  - there are no free pages left (and kswapd hasn't found new ones)
- keep the ratio user-page:swap-cache-page at about 2:1 so that
  swap-cache pages get a proper chance for aging, instead of being
  discarded immediately (hmm, why not put untouched user-pages in
  the swap cache immediately?)

For more improvements, we could use Ben's pte_list <name?>
patch so we could force-free bigorder areas and run somewhat
more efficiently.

Rik.
+-----------------------------+------------------------------+
| For Linux mm-patches, go to | "I'm busy managing memory.." |
| my homepage (via LinuxHQ).  | H.H.vanRiel@fys.ruu.nl       |
| ...submissions welcome...   | http://www.fys.ruu.nl/~riel/ |
+-----------------------------+------------------------------+

Re: PATCH: Swap shared pages (was: How to read-protect a vm_area?)

[Stephen C. Tweedie]

Hi,

On Tue, 24 Feb 1998 10:42:48 +0100 (MET), Rik van Riel
<H.H.vanRiel@fys.ruu.nl> said:

> [linux-kernel trimmed from f-ups]
> On Mon, 23 Feb 1998, Stephen C. Tweedie wrote:

>> The patch below, against 2.1.88, adds a bunch of new functionality to
>> the swapper.  The main changes are:
>> 
>> * All swapping goes through the swap cache (aka. page cache) now.

> Does this mean that _after_ the pages are properly aged
> as user-pages, they'll be aged again as page-cache pages?
> (when proper aging is added to the page cache, by eg. my patch)

No --- the swap cache is using the same data structures as the page
cache, but mainly to get lookup of swap entries still in physical
memory.  The swapout code does not leave swapped pages around in memory
unnecessarily (although it does leave the door open to performing
readahead of swap, which _would_ look very much like the current page
cache readahead and would be reclaimed by shrink_mmap()).

The page cache swapout creates a page cache association for a page when
swapping begins, and clears the link when the swapping is finished.  The
swap cache does not linger.

> I think it might be far better to:
> - put user-pages in the swap cache after they haven't been used
>   for two aging rounds
> - free swap-cache pages and page-cache pages after they haven't
>   been used for eight aging rounds (so the real aging and waiting
>   takes place here)
> - use right-shift aging here {age << 1; if(touched) age |= 0x80}
> - adapt the get_free_pages so it can allocate clean page-cache and
>   swap-cache pages when:
>   - a bigorder area can't be found
>   - there are no free pages left (and kswapd hasn't found new ones)

That is already scheduled as part of phase 4 of this work.  The patch I
have just posted is phase 2, modifying the swapper for shared pages.
Phase three is to implement MAP_SHARED | MAP_ANONYMOUS, and part four is
to do much what you describe, proactively soft-swapping data out
into the swap cache up to a predefined limit, and allowing get_free_page
to reclaim these pages atomically even from within an interrupt.  I have
already begun the work of spin-irq-locking the relevant page cache
structures.

> For more improvements, we could use Ben's pte_list <name?>
> patch so we could force-free bigorder areas and run somewhat
> more efficiently.

Ben has already been talking about some similar ideas, and I think that
yes, we do want to upgrade the swapout policy layer to work on a
physical page basis, using the pte_list walking to do its work.  The
swap cache mechanism will still be needed to perform readahead and
writeahead, but the only reason I have had to integrate it so tightly
into the policy code right now is that without pte-walking there is no
other way to properly keep pages shared over swapping.

Ideally, we really want to have just one function walking over physical
pages for reclamation, and that function should be able to deal with
swap/filemap pages, page/swap cache, buffer cache and SysV shm pages as
they come up.  I think that is achievable without too much more work
now, and it ought to give us much better performance from the swapper.

Cheers,
 Stephen.

Re: PATCH: Swap shared pages (was: How to read-protect a vm_area?)

[Rik van Riel]

On Tue, 24 Feb 1998, Stephen C. Tweedie wrote:

> That is already scheduled as part of phase 4 of this work.  The patch I
> have just posted is phase 2, modifying the swapper for shared pages.
> Phase three is to implement MAP_SHARED | MAP_ANONYMOUS, and part four is
> to do much what you describe, proactively soft-swapping data out

Hmm, is there anything I can do to help with this, or
will that just confuse things ? :-(
If not, I'll be working on buffer/cache memory limits
so one file/process can't clog up all of memory (a'la
badblocks -w), of course with DU like tunability...

Rik.
+-----------------------------+------------------------------+
| For Linux mm-patches, go to | "I'm busy managing memory.." |
| my homepage (via LinuxHQ).  | H.H.vanRiel@fys.ruu.nl       |
| ...submissions welcome...   | http://www.fys.ruu.nl/~riel/ |
+-----------------------------+------------------------------+

Re: PATCH: Swap shared pages (was: How to read-protect a vm_area?)

[Stephen C. Tweedie]

Hi,

On Wed, 25 Feb 1998 11:41:20 +0100 (MET), Rik van Riel
<H.H.vanRiel@fys.ruu.nl> said:

> On Tue, 24 Feb 1998, Stephen C. Tweedie wrote:
>> That is already scheduled as part of phase 4 of this work.  The patch I
>> have just posted is phase 2, modifying the swapper for shared pages.
>> Phase three is to implement MAP_SHARED | MAP_ANONYMOUS, and part four is
>> to do much what you describe, proactively soft-swapping data out

> Hmm, is there anything I can do to help with this

Probably not right now.  I'm probably going to swap round bits 3 and 4
of this work and defer the shared mapping, because Ben's work with
using inodes to label mm structs for anonymous maps is probably going
to be a lot better than my own plan of using a label inode per new
anonymous vma.  Ben, any thoughts about integrating this stuff or
sharing patches?  I'd like to see what you've done with this before
storming off on my own. 

> If not, I'll be working on buffer/cache memory limits
> so one file/process can't clog up all of memory (a'la
> badblocks -w), of course with DU like tunability...

Interestingly enough, the thing I wanted to do next with the VM was
similar --- implementing proper control of process RSS.  There's no
reason why we can't give big processes a smaller RSS if we start
getting memory contention, and that will let the swapper efficiently
deal with overly large processes without massively impacting the
performance of smaller processes.  Similarly, we ought to be able to
give small processes a guaranteed RSS to allow them to proceed (even
if at a reduced pace) during a swap storm.  

Doing something similar with the buffer cache will help some things a
_lot_.  Another thing to think about is to do similar tuning on the
device request lists, as I suspect that a lot of our performance /
fairness problems under high load come largely from request
starvation.  One thing I was thinking about was the possibility of
forcing processes to wait for a certain number of requests to complete
if they fill up the request queue, effectively giving them request
"credits" similar to the scheduler's credits.  This would be a simple
but probably quite effective way of making sure that processes doing
small amounts of single-block IO don't get overly starved by processes
performing large writes (such as bdflush).

Feel free to comment; I won't be working on this any time in the
immediate future...

--Stephen.

Re: PATCH: Swap shared pages (was: How to read-protect a vm_area?)

[Rik van Riel]

On Wed, 25 Feb 1998, Stephen C. Tweedie wrote:

> Feel free to comment; I won't be working on this any time in the
> immediate future...

OK, then I'll focus on memory balancing, starting with
the following simple rules:
- buffer memory isn't allowed to grow larger than
  twice the size of the pagecache when nr_free_pages < free_pages_high
- if a cached inode uses more than half of the pagecache, and
  the pagecache is larger than 1/4th of memory, and
  nr_free_pages < 2 * free_pages_high (pfew!), then we won't
  allocate new pagecache memory to satisfy _that_ inode's demand,
  but steal memory from the pagecache or buffer instead.
- do some form of RSS balancing (later on, after we get the
  stats right again).
- document the files in /proc/sys/vm and /proc/sys/kernel
  (I've started, but really should finish the files tonight :-)

Rik.
+-----------------------------+------------------------------+
| For Linux mm-patches, go to | "I'm busy managing memory.." |
| my homepage (via LinuxHQ).  | H.H.vanRiel@fys.ruu.nl       |
| ...submissions welcome...   | http://www.fys.ruu.nl/~riel/ |
+-----------------------------+------------------------------+

Re: PATCH: Swap shared pages (was: How to read-protect a vm_area?)

[Thomas Sailer]

Stephen

For the sound driver we need some way to postpone driver
shutdown until the last mmap to driver memory is unmapped
(or alternatively to force unmapping on driver close).
Could you or anyone else of the linux-mm community provide
the necessary hook in the linux mm layer? This is one
of the nasty problems with the current sound driver
that should IMHO be fixed before 2.2...

Thanks

Tom

Re: PATCH: Swap shared pages (was: How to read-protect a vm_area?)

[Stephen C. Tweedie]

On Tue, 24 Feb 1998 15:31:37 +0000 (GMT), Stephane Casset
<sept@renass3.u-strasbg.fr> said:

>> The patch below, against 2.1.88, adds a bunch of new functionality to
>> the swapper.  The main changes are:

> I tried it but got the following message :
> ipc/ipc.o: In function `shm_swap_in':
> ipc/ipc.o(.text+0x37e4): undefined reference to `read_swap_page'
> ipc/ipc.o: In function `shm_swap':
> ipc/ipc.o(.text+0x3b57): undefined reference to `write_swap_page'
> make: *** [vmlinux] Error 1

The diff below includes a patch against ipc/shm.c was missing from my
first post, and another fix for spurious warnings about shared dirty
pages.

Cheers,
 Stephen.
----------------------------------------------------------------
Index: ipc/shm.c
===================================================================
RCS file: /home/rcs/CVS/kswap3/linux/ipc/shm.c,v
retrieving revision 1.1
retrieving revision 1.2
diff -u -r1.1 -r1.2
--- shm.c	1998/02/24 08:50:30	1.1
+++ shm.c	1998/02/24 08:51:37	1.2
@@ -689,7 +689,7 @@
 			goto done;
 		}
 		if (!pte_none(pte)) {
-			read_swap_page(pte_val(pte), (char *) page);
+			rw_swap_page_nocache(READ, pte_val(pte), (char *)page);
 			pte = __pte(shp->shm_pages[idx]);
 			if (pte_present(pte))  {
 				free_page (page); /* doesn't sleep */
@@ -820,7 +820,7 @@
 	if (atomic_read(&mem_map[MAP_NR(pte_page(page))].count) != 1)
 		goto check_table;
 	shp->shm_pages[idx] = swap_nr;
-	write_swap_page (swap_nr, (char *) pte_page(page));
+	rw_swap_page_nocache (WRITE, swap_nr, (char *) pte_page(page));
 	free_page(pte_page(page));
 	swap_successes++;
 	shm_swp++;
Index: mm/page_io.c
===================================================================
RCS file: /home/rcs/CVS/kswap3/linux/mm/page_io.c,v
retrieving revision 1.4
diff -u -r1.4 page_io.c
--- page_io.c	1998/02/23 22:14:27	1.4
+++ page_io.c	1998/02/24 09:28:08
@@ -201,7 +201,9 @@
 	}
 	page->inode = &swapper_inode;
 	page->offset = entry;
+	atomic_inc(&page->count);	/* Protect from shrink_mmap() */
 	rw_swap_page(rw, entry, buffer, 1);
+	atomic_dec(&page->count);
 	page->inode = 0;
 	clear_bit(PG_swap_cache, &page->flags);
 }
Index: mm/vmscan.c
===================================================================
RCS file: /home/rcs/CVS/kswap3/linux/mm/vmscan.c,v
retrieving revision 1.5
diff -u -r1.5 vmscan.c
--- vmscan.c	1998/02/23 22:14:28	1.5
+++ vmscan.c	1998/02/24 09:22:47
@@ -108,18 +108,16 @@
 	 *
 	 * -- Stephen Tweedie 1998 */
 
-	if (pte_write(pte)) {
-		/* 
-		 * We _will_ allow dirty cached mappings later on, once
-		 * MAP_SHARED|MAP_ANONYMOUS is working, but for now
-		 * catch this as a bug.
-		 */
-		if (is_page_shared(page_map)) {
-			printk ("VM: Found a shared writable dirty page!\n");
+	if (PageSwapCache(page_map)) {
+		if (pte_write(pte)) {
+			printk ("VM: Found a writable swap-cached page!\n");
 			return 0;
 		}
-		if (PageSwapCache(page_map)) {
-			printk ("VM: Found a writable swap-cached page!\n");
+		/* We _will_ allow dirty cached mappings later
+		 * on, once MAP_SHARED|MAP_ANONYMOUS is working,
+		 * but for now catch this as a bug.  */
+		if (is_page_shared(page_map)) {
+			printk ("VM: Found a shared writable dirty page!\n");
 			return 0;
 		}
 	}
----------------------------------------------------------------