linux-ext4 - Re: [PATCH v6 01/10] mm: add zone device coherent type memory support

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Date:   Fri, 18 Feb 2022 14:20:45 -0500
From:   Felix Kuehling <felix.kuehling@....com>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     David Hildenbrand <david@...hat.com>,
        Alistair Popple <apopple@...dia.com>,
        Christoph Hellwig <hch@....de>,
        Alex Sierra <alex.sierra@....com>, akpm@...ux-foundation.org,
        linux-mm@...ck.org, rcampbell@...dia.com,
        linux-ext4@...r.kernel.org, linux-xfs@...r.kernel.org,
        amd-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        jglisse@...hat.com, willy@...radead.org
Subject: Re: [PATCH v6 01/10] mm: add zone device coherent type memory support

Am 2022-02-17 um 19:19 schrieb Jason Gunthorpe:
> On Thu, Feb 17, 2022 at 04:12:20PM -0500, Felix Kuehling wrote:
>
>> I'm thinking of a more theoretical approach: Instead of auditing all users,
>> I'd ask, what are the invariants that a vm_normal_page should have. Then
>> check, whether our DEVICE_COHERENT pages satisfy them. But maybe the concept
>> of a vm_normal_page isn't defined clearly enough for that.
> I would say the expectation is that only 'page cache and anon' pages
> are returned - ie the first union in struct page
>
> This is because the first file in your list I looked at:
>
> static int madvise_cold_or_pageout_pte_range(pmd_t *pmd,
> 				unsigned long addr, unsigned long end,
> 				struct mm_walk *walk)
>
> {
> 		page = vm_normal_page(vma, addr, ptent);
> [..]
> 		if (pageout) {
> 			if (!isolate_lru_page(page)) {
>
> Uses the LRU field, so this is incompatible with all the other page
> types.
>
> One mitigation of this might be to formally make vm_normal_page() ==
> 'pte to page cache and anon page' and add a new function that is 'pte
> to any struct page'
>
> Then go through and sort callers as appropriate.
>
> The 'pte to page cache and anon page' can detect ZONE_DEVICE by
> calling is_zone_device_page() insted of pte_devmap() and then continue
> to return NULL. This same trick will fix GUP_fast.

Sounds good to me. What about vm_normal_page_pmd? Should we remove the 
pmd_devmap check from that function as well. I'm not even sure what a 
huge zone_device page would look like, but maybe that's a worthwhile 
future optimization for our driver.

I'd propose the function names vm_normal_page and 
vm_normal_or_device_page for the two functions you described. The latter 
would basically be the current vm_normal_page with the pte_devmap check 
removed. vm_normal_page could be implemented as a wrapper around 
vm_normal_or_device_page, which just adds the !is_zone_device_page() check.

Regards,
   Felix


>
> Jason
>