From: Hugh Dickins <hughd@google.com>
To: "Kirill A. Shutemov" <kirill@shutemov.name>
Cc: Android Kernel Team <kernel-team@android.com>,
Jan Kara <jack@suse.cz>, Minchan Kim <minchan@kernel.org>,
Linus Torvalds <torvalds@linux-foundation.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Matthew Wilcox <willy@infradead.org>,
Linux-MM <linux-mm@kvack.org>,
Vinayak Menon <vinmenon@codeaurora.org>,
Linux ARM <linux-arm-kernel@lists.infradead.org>,
Catalin Marinas <catalin.marinas@arm.com>,
Andrew Morton <akpm@linux-foundation.org>,
Will Deacon <will@kernel.org>,
"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Subject: Re: [PATCH 1/2] mm: Allow architectures to request 'old' entries when prefaulting
Date: Wed, 23 Dec 2020 20:04:54 -0800 (PST) [thread overview]
Message-ID: <alpine.LSU.2.11.2012231905300.5723@eggly.anvils> (raw)
In-Reply-To: <20201222100047.p5zdb4ghagncq2oe@box>
On Tue, 22 Dec 2020, Kirill A. Shutemov wrote:
>
> Updated patch is below.
>
> From 0ec1bc1fe95587350ac4f4c866d6482383740b36 Mon Sep 17 00:00:00 2001
> From: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
> Date: Sat, 19 Dec 2020 15:19:23 +0300
> Subject: [PATCH] mm: Cleanup faultaround and finish_fault() codepaths
>
> alloc_set_pte() has two users with different requirements: in the
> faultaround code, it called from an atomic context and PTE page table
> has to be preallocated. finish_fault() can sleep and allocate page table
> as needed.
>
> PTL locking rules are also strange, hard to follow and overkill for
> finish_fault().
>
> Let's untangle the mess. alloc_set_pte() has gone now. All locking is
> explicit.
>
> The price is some code duplication to handle huge pages in faultaround
> path, but it should be fine, having overall improvement in readability.
>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
It's not ready yet.
I won't pretend to have reviewed, but I did try applying and running
with it: mostly it seems to work fine, but turned out to be leaking
huge pages (with vmstat's thp_split_page_failed growing bigger and
bigger as page reclaim cannot get rid of them).
Aside from the actual bug, filemap_map_pmd() seems suboptimal at
present: comments below (plus one comment in do_anonymous_page()).
> diff --git a/mm/filemap.c b/mm/filemap.c
> index 0b2067b3c328..f8fdbe079375 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -2831,10 +2832,74 @@ vm_fault_t filemap_fault(struct vm_fault *vmf)
> }
> EXPORT_SYMBOL(filemap_fault);
>
> +static bool filemap_map_pmd(struct vm_fault *vmf, struct page *page,
> + struct xa_state *xas)
> +{
> + struct vm_area_struct *vma = vmf->vma;
> + struct address_space *mapping = vma->vm_file->f_mapping;
> +
> + /* Huge page is mapped? No need to proceed. */
> + if (pmd_trans_huge(*vmf->pmd))
> + return true;
> +
> + if (xa_is_value(page))
> + goto nohuge;
I think it would be easier to follow if filemap_map_pages() never
passed this an xa_is_value(page): probably just skip them in its
initial xas_next_entry() loop.
> +
> + if (!pmd_none(*vmf->pmd))
> + goto nohuge;
Then at nohuge it unconditionally takes pmd_lock(), finds !pmd_none,
and unlocks again: unnecessary overhead I believe we did not have before.
> +
> + if (!PageTransHuge(page) || PageLocked(page))
> + goto nohuge;
So if PageTransHuge, but someone else temporarily holds PageLocked,
we insert a page table at nohuge, sadly preventing it from being
mapped here later by huge pmd.
> +
> + if (!page_cache_get_speculative(page))
> + goto nohuge;
> +
> + if (page != xas_reload(xas))
> + goto unref;
> +
> + if (!PageTransHuge(page))
> + goto unref;
> +
> + if (!PageUptodate(page) || PageReadahead(page) || PageHWPoison(page))
> + goto unref;
> +
> + if (!trylock_page(page))
> + goto unref;
> +
> + if (page->mapping != mapping || !PageUptodate(page))
> + goto unlock;
> +
> + if (xas->xa_index >= DIV_ROUND_UP(i_size_read(mapping->host), PAGE_SIZE))
> + goto unlock;
> +
> + do_set_pmd(vmf, page);
Here is the source of the huge page leak: do_set_pmd() can fail
(and we would do better to have skipped most of its failure cases long
before getting this far). It worked without leaking once I patched it:
- do_set_pmd(vmf, page);
- unlock_page(page);
- return true;
+ if (do_set_pmd(vmf, page) == 0) {
+ unlock_page(page);
+ return true;
+ }
> + unlock_page(page);
> + return true;
> +unlock:
> + unlock_page(page);
> +unref:
> + put_page(page);
> +nohuge:
> + vmf->ptl = pmd_lock(vma->vm_mm, vmf->pmd);
> + if (likely(pmd_none(*vmf->pmd))) {
> + mm_inc_nr_ptes(vma->vm_mm);
> + pmd_populate(vma->vm_mm, vmf->pmd, vmf->prealloc_pte);
> + vmf->prealloc_pte = NULL;
> + }
> + spin_unlock(vmf->ptl);
I think it's a bit weird to hide this page table insertion inside
filemap_map_pmd() (I guess you're thinking that this function deals
with pmd level, but I'd find it easier to have a filemap_map_huge()
dealing with the huge mapping). Better to do it on return into
filemap_map_pages(); maybe filemap_map_pmd() or filemap_map_huge()
would then need to return vm_fault_t rather than bool, I didn't try.
> +
> + /* See comment in handle_pte_fault() */
> + if (pmd_devmap_trans_unstable(vmf->pmd))
> + return true;
> +
> + return false;
> +}
...
> diff --git a/mm/memory.c b/mm/memory.c
> index c48f8df6e502..96d62774096a 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -3490,7 +3490,7 @@ static vm_fault_t do_anonymous_page(struct vm_fault *vmf)
> if (pte_alloc(vma->vm_mm, vmf->pmd))
> return VM_FAULT_OOM;
>
> - /* See the comment in pte_alloc_one_map() */
> + /* See the comment in map_set_pte() */
No, no such function: probably should be like the others and say
/* See comment in handle_pte_fault() */
Hugh
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next prev parent reply other threads:[~2020-12-24 4:07 UTC|newest]
Thread overview: 55+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-12-09 16:39 [PATCH 0/2] Create 'old' ptes for faultaround mappings on arm64 with hardware access flag Will Deacon
2020-12-09 16:39 ` [PATCH 1/2] mm: Allow architectures to request 'old' entries when prefaulting Will Deacon
2020-12-09 17:58 ` Linus Torvalds
2020-12-09 18:40 ` Will Deacon
2020-12-09 19:04 ` Linus Torvalds
2020-12-09 20:32 ` Matthew Wilcox
2020-12-09 21:04 ` Linus Torvalds
2020-12-10 15:08 ` Kirill A. Shutemov
2020-12-10 17:23 ` Linus Torvalds
2020-12-14 16:07 ` Kirill A. Shutemov
2020-12-14 17:54 ` Linus Torvalds
2020-12-14 18:56 ` Matthew Wilcox
2020-12-16 17:07 ` Kirill A. Shutemov
2020-12-16 18:41 ` Linus Torvalds
2020-12-17 10:54 ` Kirill A. Shutemov
2020-12-17 18:22 ` Linus Torvalds
2020-12-18 11:04 ` Kirill A. Shutemov
2020-12-18 18:56 ` Linus Torvalds
2020-12-19 12:41 ` Kirill A. Shutemov
2020-12-19 20:08 ` Linus Torvalds
2020-12-19 20:34 ` Linus Torvalds
2020-12-22 10:00 ` Kirill A. Shutemov
2020-12-24 4:04 ` Hugh Dickins [this message]
2020-12-25 11:31 ` Kirill A. Shutemov
2020-12-26 17:57 ` Linus Torvalds
2020-12-26 20:43 ` Kirill A. Shutemov
2020-12-26 21:03 ` Hugh Dickins
2020-12-26 21:16 ` Linus Torvalds
2020-12-26 22:40 ` Kirill A. Shutemov
2020-12-27 0:45 ` Hugh Dickins
2020-12-27 2:38 ` Hugh Dickins
2020-12-27 19:38 ` Linus Torvalds
2020-12-27 20:32 ` Damian Tometzki
2020-12-27 22:35 ` Hugh Dickins
2020-12-27 23:12 ` Linus Torvalds
2020-12-27 23:40 ` Linus Torvalds
2020-12-27 23:55 ` Kirill A. Shutemov
2020-12-27 23:48 ` Kirill A. Shutemov
2020-12-28 1:54 ` Linus Torvalds
2020-12-28 6:43 ` Hugh Dickins
2020-12-28 12:53 ` Kirill A. Shutemov
2020-12-28 18:47 ` Linus Torvalds
2020-12-28 21:58 ` Linus Torvalds
2020-12-29 13:28 ` Kirill A. Shutemov
2020-12-29 15:19 ` Matthew Wilcox
2020-12-29 20:52 ` Linus Torvalds
2020-12-28 22:05 ` Kirill A. Shutemov
2020-12-28 22:12 ` Kirill A. Shutemov
2020-12-29 4:35 ` Hugh Dickins
2020-12-28 23:28 ` Linus Torvalds
2020-12-26 21:07 ` Linus Torvalds
2020-12-26 21:41 ` Matthew Wilcox
2020-12-09 16:39 ` [PATCH 2/2] arm64: mm: Implement arch_wants_old_faultaround_pte() Will Deacon
2020-12-09 18:35 ` Catalin Marinas
2020-12-09 18:46 ` Will Deacon
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