linux-ext4 - Re: [PATCH v1 13/15] mm: handling Non-LRU pages returned by vm_normal

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Date:   Thu, 12 May 2022 17:33:44 -0500
From:   "Sierra Guiza, Alejandro (Alex)" <alex.sierra@....com>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     david@...hat.com, Felix.Kuehling@....com, 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, hch@....de, jglisse@...hat.com,
        apopple@...dia.com, willy@...radead.org, akpm@...ux-foundation.org
Subject: Re: [PATCH v1 13/15] mm: handling Non-LRU pages returned by
 vm_normal_pages


On 5/11/2022 1:50 PM, Jason Gunthorpe wrote:
> On Thu, May 05, 2022 at 04:34:36PM -0500, Alex Sierra wrote:
>
>> diff --git a/mm/memory.c b/mm/memory.c
>> index 76e3af9639d9..892c4cc54dc2 100644
>> +++ b/mm/memory.c
>> @@ -621,6 +621,13 @@ struct page *vm_normal_page(struct vm_area_struct *vma, unsigned long addr,
>>   		if (is_zero_pfn(pfn))
>>   			return NULL;
>>   		if (pte_devmap(pte))
>> +/*
>> + * NOTE: Technically this should goto check_pfn label. However, page->_mapcount
>> + * is never incremented for device pages that are mmap through DAX mechanism
>> + * using pmem driver mounted into ext4 filesystem. When these pages are unmap,
>> + * zap_pte_range is called and vm_normal_page return a valid page with
>> + * page_mapcount() = 0, before page_remove_rmap is called.
>> + */
>>   			return NULL;
> ? Where does this series cause device coherent to be returned?
In our case, device coherent pages could be obtained as a result of 
migration(Patches 6/7 of 15), ending up mapped in CPU page tables. Later 
on, these pages might need to be returned by get_user_pages or other 
callers through vm_normal_pages. Our approach in this series, is to 
handle device-coherent-managed pages returned by vm_normal_pages, inside 
each caller. EX. device coherent pages don’t support LRU lists, NUMA 
migration or THP.
>
> Wasn't the plan to not set pte_devmap() ?

amdgpu does not set pte_devmap for our DEVICE_COHERENT pages. DEVMAP 
flags are set by drivers like virtio_fs or pmem, where 
MEMORY_DEVICE_FS_DAX type is used.
This patch series deals with DEVICE_COHERENT pages. My understanding 
was, that the DAX code and DEVICE_GENERIC would be fixed up later by 
someone more familiar with it. Were you expecting that we'd fix the DAX 
usage of pte_devmap flags in this patch series as well?

Regards,
Alex Sierra

>
> Jason