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From: "Thomas Hellström (VMware)" <thomas_os@shipmail.org>
To: Linus Torvalds <torvalds@linux-foundation.org>,
	Thomas Hellstrom <thellstrom@vmware.com>
Cc: "Kirill A. Shutemov" <kirill@shutemov.name>,
	"Linux Kernel Mailing List" <linux-kernel@vger.kernel.org>,
	Linux-MM <linux-mm@kvack.org>,
	"Matthew Wilcox" <willy@infradead.org>,
	"Will Deacon" <will.deacon@arm.com>,
	"Peter Zijlstra" <peterz@infradead.org>,
	"Rik van Riel" <riel@surriel.com>,
	"Minchan Kim" <minchan@kernel.org>,
	"Michal Hocko" <mhocko@suse.com>,
	"Huang Ying" <ying.huang@intel.com>,
	"Jérôme Glisse" <jglisse@redhat.com>
Subject: Re: [PATCH v4 3/9] mm: pagewalk: Don't split transhuge pmds when a pmd_entry is present
Date: Wed, 9 Oct 2019 22:06:32 +0200	[thread overview]
Message-ID: <c054849e-1e24-6b27-6a54-740ea9d17054@shipmail.org> (raw)
In-Reply-To: <CAHk-=wgy-ULe8UmEDn9gCCmTtw65chS0h309WrTaQhK3RAXM-A@mail.gmail.com>

On 10/9/19 9:20 PM, Linus Torvalds wrote:
>
> No. Your logic is garbage. The above code is completely broken.
>
> YOU CAN NOT AVOID TRHE SPLIT AND THEN GO ON AT THE PTE LEVEL.
>
> Don't you get it? There *is* no PTE level if you didn't split.

Hmm, This paragraph makes me think we have very different perceptions about what I'm trying to achieve.

I wanted the pte level to *only* get called for *pre-existing* pte entries.
Surely those must be able to exist even if we don't split occasional huge pmds in the pagewalk code?

>
> So what you should do is to just always return 0 in your pmd_entry().
> Boom, done. The only reason for the pmd_entry existing at all is to
> get the warning. Then, if you don't want to split it, you make that
> warning just return an error (or a positive value) instead and say
> "ok, that was bad, we don't handle it at all".
>
> And in some _future_ life, if anybody wants to actually say "yeah,
> let's not split it", make it have some "yeah I handled it" case.

Well yes, this is exactly what I want. Because any huge pmd we encounter 
should be read-only.

/Thomas



  reply	other threads:[~2019-10-09 20:06 UTC|newest]

Thread overview: 32+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-10-08  9:14 [PATCH v4 0/9] Emulated coherent graphics memory take 2 Thomas Hellström (VMware)
2019-10-08  9:15 ` [PATCH v4 1/9] mm: Remove BUG_ON mmap_sem not held from xxx_trans_huge_lock() Thomas Hellström (VMware)
2019-10-08  9:15 ` [PATCH v4 2/9] mm: pagewalk: Take the pagetable lock in walk_pte_range() Thomas Hellström (VMware)
2019-10-09 15:14   ` Kirill A. Shutemov
2019-10-09 16:07     ` Linus Torvalds
2019-10-08  9:15 ` [PATCH v4 3/9] mm: pagewalk: Don't split transhuge pmds when a pmd_entry is present Thomas Hellström (VMware)
2019-10-09 15:27   ` Kirill A. Shutemov
2019-10-09 15:27     ` Kirill A. Shutemov
2019-10-09 16:20     ` Thomas Hellström (VMware)
2019-10-09 16:20       ` Thomas Hellström (VMware)        
2019-10-09 16:21     ` Linus Torvalds
2019-10-09 17:03       ` Thomas Hellström (VMware)
2019-10-09 17:16         ` Linus Torvalds
2019-10-09 18:52           ` Thomas Hellstrom
2019-10-09 19:20             ` Linus Torvalds
2019-10-09 20:06               ` Thomas Hellström (VMware) [this message]
2019-10-09 20:20                 ` Linus Torvalds
2019-10-09 22:30                   ` Thomas Hellström (VMware)
2019-10-09 23:50                     ` Thomas Hellström (VMware)
2019-10-09 23:51                     ` Linus Torvalds
2019-10-10  0:18                       ` Linus Torvalds
2019-10-10  1:09                       ` Thomas Hellström (VMware)
2019-10-10  2:07                         ` Linus Torvalds
2019-10-10  6:15                           ` Thomas Hellström (VMware)
2019-10-08  9:15 ` [PATCH v4 4/9] mm: Add a walk_page_mapping() function to the pagewalk code Thomas Hellström (VMware)
2019-10-08  9:15 ` [PATCH v4 5/9] mm: Add write-protect and clean utilities for address space ranges Thomas Hellström (VMware)
2019-10-08 17:06   ` Linus Torvalds
2019-10-08 18:25     ` Thomas Hellstrom
2019-10-08  9:15 ` [PATCH v4 6/9] drm/vmwgfx: Implement an infrastructure for write-coherent resources Thomas Hellström (VMware)
2019-10-08  9:15 ` [PATCH v4 7/9] drm/vmwgfx: Use an RBtree instead of linked list for MOB resources Thomas Hellström (VMware)
2019-10-08  9:15 ` [PATCH v4 8/9] drm/vmwgfx: Implement an infrastructure for read-coherent resources Thomas Hellström (VMware)
2019-10-08  9:15 ` [PATCH v4 9/9] drm/vmwgfx: Add surface dirty-tracking callbacks Thomas Hellström (VMware)

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