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SubjectVM_RESERVED [was Re: mapping user space buffer to kernel address space]
On Thu, Oct 19, 2000 at 03:45:43PM -0700, Linus Torvalds wrote:
>
>
> On Fri, 20 Oct 2000, Andrea Arcangeli wrote:
>
> > On Thu, Oct 19, 2000 at 05:16:14PM -0400, Jeff Garzik wrote:
> > > solution is really elegant. Excluding all the debug code and assertions
> > > I stick in there, the guts of via audio mmap support went from ~50 lines
> > > to ~10.
> >
> > Was it 50 lines with remap_page_range?
>
> You cannot use remap_page_range() if the pages aren't physically
> contiguous.

Yep.

And about the other question?

> Face it, Andrea, remap_page_range() is a very limited hack that only works
> for a very limited subset of the things that real people want to do. It is
> _not_ a generic solution, while the nopage() approach _is_ generic.

But without keeping those pages pinned in the ptes, the nopage approch is
_suboptimal_ and adding a dummy swapout that returns "progress" where no
progress is been made will also _break_ the VM with relatively large device
mmapped vmas. It would at least _certainly_ break 2.2.18pre15aa1 (if 2.4.x has
an oom deadlock in GFP it won't break of course).

I'm fine to drop remap_page_range and the PG_reserved bit, but to do that I'd
suggest to add a new per-VMA VM_RESERVED bitflags. This will also avoid to walk
pagetables where it makes no sense to unmap the pages (it won't only avoid
senseless page faults) and it will fix the VM callbacks problem that returns
"success" when no progress is made. (SHM returns a dummy 0 too, but progress is
been made in that case)

The only reason it could make sense to do that unmapping is if we would be able
to also free the _pagetables_, but we do that only via munmap if a whole gdt
entry gets unmapped (and after munmap the page is been just unmapped anyways).
So once we'll free pagetables via swap_out (maybe never) some driver with
houndred of mbytes of virtual mapping may choose to drop the VM_RESERVED
bitflag from their vmas if pagefaults aren't a performance problem for that
drivers.

I just find hard to widespread propose something that ends doing the wrong
thing (ok I see it's not a big deal but it's still the _wrong_ thing to do and
the VM_RESERVED way will fix it a _zero_ cost, it will improve performance
compared to PG_reserved and it will also avoid the uglyness of having all the
SG drivers out there implementing a dummy swapout callback :).

This introduces VM_RESERVED against test10-pre4:

--- 2.4.0-test10-pre4/include/linux/mm.h.~1~ Fri Oct 20 17:56:09 2000
+++ 2.4.0-test10-pre4/include/linux/mm.h Fri Oct 20 18:23:47 2000
@@ -95,6 +95,7 @@

#define VM_DONTCOPY 0x00020000 /* Do not copy this vma on fork */
#define VM_DONTEXPAND 0x00040000 /* Cannot expand with mremap() */
+#define VM_RESERVED 0x00080000 /* Don't unmap it from swap_out */

#define VM_STACK_FLAGS 0x00000177

--- 2.4.0-test10-pre4/include/linux/wrapper.h.~1~ Tue Aug 8 06:01:36 2000
+++ 2.4.0-test10-pre4/include/linux/wrapper.h Fri Oct 20 18:26:02 2000
@@ -29,8 +29,17 @@
#define vma_get_end(v) v->vm_end
#define vma_get_page_prot(v) v->vm_page_prot

+/*
+ * mem_map_reserve()/unreserve() are going to be obsoleted by
+ * vma_reserve(). (unreserve shouldn't be necessary)
+ *
+ * Instead of marking the pages as reserved, just mark the vma as reserved
+ * this will improve performance (it's zero cost unlike the PG_reserved check)
+ * and it will be trivial for not physically contigous mappings too.
+ */
#define mem_map_reserve(p) set_bit(PG_reserved, &p->flags)
#define mem_map_unreserve(p) clear_bit(PG_reserved, &p->flags)
#define mem_map_inc_count(p) atomic_inc(&(p->count))
#define mem_map_dec_count(p) atomic_dec(&(p->count))
+#define vma_reserve(vma) ((vma)->vm_flags |= VM_RESERVED)
#endif
--- 2.4.0-test10-pre4/mm/vmscan.c.~1~ Fri Oct 20 17:56:10 2000
+++ 2.4.0-test10-pre4/mm/vmscan.c Fri Oct 20 18:11:09 2000
@@ -318,7 +318,7 @@
unsigned long end;

/* Don't swap out areas which are locked down */
- if (vma->vm_flags & VM_LOCKED)
+ if (vma->vm_flags & (VM_LOCKED|VM_RESERVED))
return 0;

pgdir = pgd_offset(mm, address);

And this shows a pratical (but untested :) demonstration of the VM_RESERVED
usage:

--- 2.4.0-test10-pre4/drivers/media/video/bttv-driver.c.~1~ Thu Oct 12 03:04:42 2000
+++ 2.4.0-test10-pre4/drivers/media/video/bttv-driver.c Fri Oct 20 18:32:43 2000
@@ -39,7 +39,6 @@
#include <linux/sched.h>
#include <asm/segment.h>
#include <linux/types.h>
-#include <linux/wrapper.h>
#include <linux/interrupt.h>
#include <linux/kmod.h>
#include <linux/vmalloc.h>
@@ -181,40 +180,17 @@
static void * rvmalloc(signed long size)
{
void * mem;
- unsigned long adr, page;

mem=vmalloc_32(size);
if (mem)
- {
memset(mem, 0, size); /* Clear the ram out, no junk to the user */
- adr=(unsigned long) mem;
- while (size > 0)
- {
- page = kvirt_to_pa(adr);
- mem_map_reserve(virt_to_page(__va(page)));
- adr+=PAGE_SIZE;
- size-=PAGE_SIZE;
- }
- }
return mem;
}

static void rvfree(void * mem, signed long size)
{
- unsigned long adr, page;
-
if (mem)
- {
- adr=(unsigned long) mem;
- while (size > 0)
- {
- page = kvirt_to_pa(adr);
- mem_map_unreserve(virt_to_page(__va(page)));
- adr+=PAGE_SIZE;
- size-=PAGE_SIZE;
- }
vfree(mem);
- }
}


--- 2.4.0-test10-pre4/drivers/media/video/videodev.c.~1~ Wed Jul 19 07:35:33 2000
+++ 2.4.0-test10-pre4/drivers/media/video/videodev.c Fri Oct 20 18:32:38 2000
@@ -27,6 +27,7 @@
#include <linux/errno.h>
#include <linux/videodev.h>
#include <linux/init.h>
+#include <linux/wrapper.h>

#include <asm/uaccess.h>
#include <asm/system.h>
@@ -228,6 +229,7 @@
int ret = -EINVAL;
struct video_device *vfl=video_device[MINOR(file->f_dentry->d_inode->i_rdev)];
if(vfl->mmap) {
+ vma_reserve(vma);
lock_kernel();
ret = vfl->mmap(vfl, (char *)vma->vm_start,
(unsigned long)(vma->vm_end-vma->vm_start));
bttv-driver.c | 24 ------------------------
videodev.c | 2 ++
2 files changed, 2 insertions(+), 24 deletions(-)
Once VM_RESERVED is in place using nopage instead of remap_page_range looks
fine to me (we'll have an additional startup page fault but that can
explained as a mmap latency reduction and after all people can keep
using remap_page_range to avoid that first page fault as the bttv
driver would still do with the above patch applied even if it's just
using the new vma_reserve way).

(in theory we could use VM_LOCKED for that but as you noticed we would
end corrupting the number of locked-page stats and I just find cleaner
to not mix mlock and reserved kernel pages issues)

Andrea
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