Re: What to expect with the 2.6 VM

Andrea Arcangeli
2003-07-02T21:49:13Z

On Wed, Jul 02, 2003 at 02:40:32PM -0700, William Lee Irwin III wrote:
> On Wed, Jul 02, 2003 at 07:47:00PM +0200, Andrea Arcangeli wrote:
> > actually other more invasive ways could be to move rmap into highmem.
> > Also the page clustering could also hide part of the mem overhead by
> > assuming the pagetables to be contiguos, but page clustering isn't part
> > of mainline yet either.
>
> BSD-style page clustering preserves virtual contiguity of a software
> page, but the new things don't; for ABI preservation, virtually
> discontiguous, partial, and misaligned mappings of pages are handled.
>
> The desired behavior can in principle be partially recovered by
> scanning within a software page size -sized "blast radius" for each
> chain element and only chaining enough elements to find the relevant
> ptes that way.
>
> As for remap_file_pages(), either people are misunderstanding or
> ignoring me. There is a lovely three-step method to handling it:
>
> (a) fix the truncate() bug; it is just a literal bug. There are at
> least 3 different ways to fix it:
> (i) tag vmas touched by remap_file_pages() for exhaustive search
> (ii) do a cleanup pass after the current vmtruncate() doing
> try_to_unmap() on any still-mapped pages
> (iii) drop the current vmtruncate() entirely and do try_to_unmap()
> on each truncated page
> (ii) and (iii) do the locks in the wrong order, so some still-
> mapped but truncated page could be out there; this could be
> handed by Yet Another Cleanup Pass that does (i) or by tolerating
> the new state elsewhere in the VM. There's plenty of ways to
> code this and a couple choices of semantics (i.e make it
> failable or reliable).
>
> (b) implement the bits omitting pte_chains for mlock()'d mappings
> This is obvious. Yank them off the LRU, set a bitflag, and
> reuse page->lru for a counter.
>
> (c) redo the logic around page_convert_anon() and incrementally build
> pte_chains for remap_file_pages().
> The anobjrmap code did exactly this, but it was chaining
> distinct user virtual addresses instead.
> (i) you always have the pte_chain in hand anyway; the core
> is always prepped to handle allocating them now
> (ii) instead of just bailing for file-backed pages in
> page_add_rmap(), pass it enough information to know
> whether the address matches what it should from the
> vma, and start chaining if it doesn't
> (iii) but you say ->mapcount sharing space with the chain is a
> problem? no, it's not; again, take a cue from anobjrmap:
> if a file-backed page needs a pte_chain, shoehorn
> ->mapcount into the first pte_chain block dangling off it
>
> After all 3 are done, remap_file_pages() integrates smoothly into the VM,
> requires no magical privileges, nothing magical or brutally invasive
> that would scare people just before 2.6.0 is required, and the big
> apps can get their magical lowmem savings by just mlock()'ing _anything_
> they do massive sharing with, regardless of remap_file_pages().
>
> Does anyone get it _now_?

the problem with the above is that it is an order of magnitude more
complicated than just providing the feature remap_file_pages is been
written for. Removing the pte_chains via mlock is trivial, but then go
ahead and rebuild it synchronously in O(N) scanning the whole 1T of
virtual address space when I munlock.

In turn I still prefer the simplest possible approch. I see no strong
reason why we should complicate the kernel like that to make
remap_file_pages generic.

IMHO remap_file_pages wouldn't exist today in the kernel if 32bit archs
would be limited to 4G of ram. It's primarly a 32bit hack and as such we
should try to get away with it with the minimal damage to the rest of
the kernel (in a way that emulator can use too though, via a sysctl or
similar).

Now releasing the pte_chain during mlock would be a generic feature
orthogonal with the above I know, but I doubt you really care about it
for all other usages (also given the nearly unfixable complexity it
would introduce in munlock).

Andrea