From: Jason Gunthorpe <jgg@ziepe.ca>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Xu <peterx@redhat.com>, John Hubbard <jhubbard@nvidia.com>,
Leon Romanovsky <leonro@nvidia.com>,
Linux-MM <linux-mm@kvack.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
"Maya B . Gokhale" <gokhale2@llnl.gov>,
Yang Shi <yang.shi@linux.alibaba.com>,
Marty Mcfadden <mcfadden8@llnl.gov>,
Kirill Shutemov <kirill@shutemov.name>,
Oleg Nesterov <oleg@redhat.com>, Jann Horn <jannh@google.com>,
Jan Kara <jack@suse.cz>, Kirill Tkhai <ktkhai@virtuozzo.com>,
Andrea Arcangeli <aarcange@redhat.com>,
Christoph Hellwig <hch@lst.de>,
Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH 1/4] mm: Trial do_wp_page() simplification
Date: Thu, 17 Sep 2020 16:38:24 -0300 [thread overview]
Message-ID: <20200917193824.GL8409@ziepe.ca> (raw)
In-Reply-To: <CAHk-=wjtfjB3TqTFRzVmOrB9Mii6Yzc-=wKq0fu4ruDE6AsJgg@mail.gmail.com>
On Thu, Sep 17, 2020 at 11:11:06AM -0700, Linus Torvalds wrote:
> (a) if the pinner is going to change the page, it will have to get
> the pin with FOLL_WRITE in addition to FOLL_PIN
>
> (b) that means it will do the COW and mark the page writable and dirty
Yep
> (c) the whole _point_ of the FOLL_PIN is that subsequent operations
> shouldn't make it non-writable any more (ie it can't be unmapped, and
> we should synchronize on fork etc)
It is the ideal, but FOLL_PIN has been troubled for a long time:
https://lwn.net/Articles/753027/
ie writeprotect is known to happen due to writeback. I had understood
that fork() will also cause write protect. Eg
copy_process()
copy_mm()
dup_mm()
dup_mmap()
copy_page_range()
[..]
copy_one_pte()
Gets to:
if (is_cow_mapping(vm_flags) && pte_write(pte)) {
ptep_set_wrprotect(src_mm, addr, src_pte);
pte = pte_wrprotect(pte);
}
Which blindly write protects a FOLL_PIN page. Here src_pte will be
under a pin.
I *think* the failing test is basically:
1) pin_user_pages(mem, FOLL_FORCE | FOLL_WRITE)
2) pid = fork()
3) child: does a bit, then exec
4) parent: waitpid(pid)
5) *mem = 0
Here #5 is the WP triggered COW. We know a WP triggered COW is
happening from the bisect and success result with MADV_DONTFORK.
#2 triggers the dup_mmap() and the ptep_set_wrprotect() (From
inspection, at least)
This "Trial do_wp_page() simplification" patch means that when #5 goes
to do COW it gets a copy instead of a re-use because the
reuse_swap_page() was aborting the copy before.
So, to your point, yes ideally FOLL_PIN would never write-protect
pages!
Looking for awhile, this now looks reasonable and
doable. page_maybe_dma_pinned() was created for exactly this kind of
case.
I've attached a dumb sketch for the pte level (surely wrong! I have
never looked at this part of the mm before!) at the end of this
message.
Peter, do you know this better? Does this inspire you to make a patch? :)
Would really love to see this. We have such a huge expensive mess with
MADV_DONTFORK, this would eliminate all that overhead.
Thanks,
Jason
diff --git a/mm/memory.c b/mm/memory.c
index 469af373ae76e1..6bc19a43da1391 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -689,6 +689,21 @@ struct page *vm_normal_page_pmd(struct vm_area_struct *vma, unsigned long addr,
}
#endif
+static int duplicate_page(pte_t *newpte, struct vm_area_struct *vma,
+ unsigned long address, struct page *page)
+{
+ struct page *new_page;
+
+ new_page = alloc_page_vma(GFP_HIGHUSER_MOVABLE, vma, address);
+ if (!new_page)
+ return -ENOMEM;
+ copy_user_highpage(new_page, page, address, vma);
+
+ /* FIXME: surely more than this */
+ *newpte = mk_pte(new_page, vma->vm_page_prot);
+ return 0;
+}
+
/*
* copy one vm_area from one task to the other. Assumes the page tables
* already present in the new task to be cleared in the whole range
@@ -703,6 +718,7 @@ copy_one_pte(struct mm_struct *dst_mm, struct mm_struct *src_mm,
unsigned long vm_flags = vma->vm_flags;
pte_t pte = *src_pte;
struct page *page;
+ bool do_src_wp;
/* pte contains position in swap or file, so copy. */
if (unlikely(!pte_present(pte))) {
@@ -775,15 +791,6 @@ copy_one_pte(struct mm_struct *dst_mm, struct mm_struct *src_mm,
goto out_set_pte;
}
- /*
- * If it's a COW mapping, write protect it both
- * in the parent and the child
- */
- if (is_cow_mapping(vm_flags) && pte_write(pte)) {
- ptep_set_wrprotect(src_mm, addr, src_pte);
- pte = pte_wrprotect(pte);
- }
-
/*
* If it's a shared mapping, mark it clean in
* the child
@@ -800,11 +807,34 @@ copy_one_pte(struct mm_struct *dst_mm, struct mm_struct *src_mm,
if (!(vm_flags & VM_UFFD_WP))
pte = pte_clear_uffd_wp(pte);
+ do_src_wp = is_cow_mapping(vm_flags) && pte_write(pte);
page = vm_normal_page(vma, addr, pte);
if (page) {
get_page(page);
page_dup_rmap(page, false);
rss[mm_counter(page)]++;
+
+ /*
+ * If a page is DMA pinned we never want to write protect it,
+ * copy it now.
+ */
+ if (do_src_wp && page_maybe_dma_pinned(page)) {
+ do_src_wp = false;
+ ret = duplicate_page(&pte, vma, addr, page);
+ if (ret)
+ /* FIXME: need to restructure a bit to handle this */
+ return ret;
+ }
+ }
+
+ /*
+ * If it's a COW mapping, write protect it both
+ * in the parent and the child
+ * FIXME check carefully this is new order is OK
+ */
+ if (do_src_wp) {
+ ptep_set_wrprotect(src_mm, addr, src_pte);
+ pte = pte_wrprotect(pte);
}
out_set_pte:
next prev parent reply other threads:[~2020-09-17 20:08 UTC|newest]
Thread overview: 98+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-08-21 23:49 [PATCH 0/4] mm: Simplfy cow handling Peter Xu
2020-08-21 23:49 ` [PATCH 1/4] mm: Trial do_wp_page() simplification Peter Xu
2020-08-24 8:36 ` Kirill Tkhai
2020-08-24 14:30 ` Jan Kara
2020-08-24 15:37 ` Kirill Tkhai
2020-08-24 18:22 ` Linus Torvalds
2020-09-01 7:01 ` Hugh Dickins
2020-09-14 14:38 ` Jason Gunthorpe
2020-09-14 17:32 ` Linus Torvalds
2020-09-14 18:34 ` Peter Xu
2020-09-14 21:15 ` Peter Xu
2020-09-14 22:55 ` Jason Gunthorpe
2020-09-14 22:59 ` Linus Torvalds
2020-09-14 23:28 ` Jason Gunthorpe
2020-09-15 0:19 ` Linus Torvalds
2020-09-15 14:50 ` Peter Xu
2020-09-15 15:17 ` Peter Xu
2020-09-15 16:05 ` Jason Gunthorpe
2020-09-15 18:29 ` Jason Gunthorpe
2020-09-15 19:13 ` Peter Xu
2020-09-15 19:38 ` Jason Gunthorpe
2020-09-15 21:33 ` Peter Xu
2020-09-15 23:22 ` Jason Gunthorpe
2020-09-16 1:50 ` John Hubbard
2020-09-16 17:48 ` Jason Gunthorpe
2020-09-16 18:46 ` Peter Xu
2020-09-17 11:25 ` Jason Gunthorpe
2020-09-17 18:11 ` Linus Torvalds
2020-09-17 19:38 ` Jason Gunthorpe [this message]
2020-09-17 19:51 ` Linus Torvalds
2020-09-18 16:40 ` Peter Xu
2020-09-18 17:16 ` Linus Torvalds
2020-09-18 19:57 ` Peter Xu
2020-09-18 17:32 ` Jason Gunthorpe
2020-09-18 20:40 ` Peter Xu
2020-09-18 20:59 ` Linus Torvalds
2020-09-19 0:28 ` Jason Gunthorpe
2020-09-18 21:06 ` John Hubbard
2020-09-19 0:01 ` Jason Gunthorpe
2020-09-21 8:35 ` Jan Kara
2020-09-21 12:03 ` Jason Gunthorpe
2022-02-16 16:59 ` Oded Gabbay
2022-02-16 17:24 ` Oded Gabbay
2022-02-16 19:04 ` Linus Torvalds
2022-02-16 19:20 ` Oded Gabbay
2022-02-16 19:24 ` David Hildenbrand
2020-09-21 13:42 ` Michal Hocko
2020-09-21 14:18 ` Peter Xu
2020-09-21 14:28 ` Michal Hocko
2020-09-21 14:38 ` Tejun Heo
2020-09-21 14:43 ` Christian Brauner
2020-09-21 14:55 ` Michal Hocko
2020-09-21 15:04 ` Christian Brauner
2020-09-21 16:06 ` Michal Hocko
2020-09-23 7:53 ` Michal Hocko
2020-09-21 14:41 ` Christian Brauner
2020-09-21 14:57 ` Michal Hocko
2020-09-21 16:31 ` Peter Xu
2020-09-17 18:14 ` Peter Xu
2020-09-17 18:26 ` Linus Torvalds
2020-09-17 19:03 ` Peter Xu
2020-09-17 19:42 ` Linus Torvalds
2020-09-17 19:55 ` John Hubbard
2020-09-17 20:06 ` Jason Gunthorpe
2020-09-17 20:19 ` John Hubbard
2020-09-17 20:25 ` Jason Gunthorpe
2020-09-17 20:35 ` Linus Torvalds
2020-09-17 21:40 ` Peter Xu
2020-09-17 22:09 ` Jason Gunthorpe
2020-09-17 22:25 ` Linus Torvalds
2020-09-17 22:48 ` Ira Weiny
2020-09-18 9:36 ` Jan Kara
2020-09-18 9:44 ` Jan Kara
2020-09-18 16:19 ` Jason Gunthorpe
2020-09-15 10:23 ` Leon Romanovsky
2020-09-15 15:56 ` Jason Gunthorpe
2020-09-15 15:03 ` Oleg Nesterov
2020-09-15 16:18 ` Peter Xu
2020-08-21 23:49 ` [PATCH 2/4] mm/ksm: Remove reuse_ksm_page() Peter Xu
2020-08-21 23:49 ` [PATCH 3/4] mm/gup: Remove enfornced COW mechanism Peter Xu
2020-09-14 14:27 ` Oleg Nesterov
2020-09-14 17:59 ` Peter Xu
2020-09-14 19:03 ` Linus Torvalds
2020-08-21 23:49 ` [PATCH 4/4] mm: Add PGREUSE counter Peter Xu
2020-08-22 16:14 ` Linus Torvalds
2020-08-24 0:24 ` Peter Xu
2020-08-22 16:05 ` [PATCH 0/4] mm: Simplfy cow handling Linus Torvalds
2020-08-23 23:58 ` Peter Xu
2020-08-24 8:38 ` Kirill Tkhai
2020-08-27 14:15 ` Peter Xu
2021-02-02 14:40 [PATCH 1/4] mm: Trial do_wp_page() simplification Gal Pressman
2021-02-02 16:31 ` Peter Xu
2021-02-02 16:44 ` Jason Gunthorpe
2021-02-02 17:05 ` Peter Xu
2021-02-02 17:13 ` Jason Gunthorpe
2021-02-03 12:43 ` Gal Pressman
2021-02-03 14:00 ` Jason Gunthorpe
2021-02-03 14:47 ` Gal Pressman
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20200917193824.GL8409@ziepe.ca \
--to=jgg@ziepe.ca \
--cc=aarcange@redhat.com \
--cc=akpm@linux-foundation.org \
--cc=gokhale2@llnl.gov \
--cc=hch@lst.de \
--cc=jack@suse.cz \
--cc=jannh@google.com \
--cc=jhubbard@nvidia.com \
--cc=kirill@shutemov.name \
--cc=ktkhai@virtuozzo.com \
--cc=leonro@nvidia.com \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-mm@kvack.org \
--cc=mcfadden8@llnl.gov \
--cc=oleg@redhat.com \
--cc=peterx@redhat.com \
--cc=torvalds@linux-foundation.org \
--cc=yang.shi@linux.alibaba.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).