linux-ext4 - Re: [PATCH v1 1/3] mm: split vm_normal_pages for LRU and non-LRU handling

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Date:   Thu, 10 Mar 2022 16:58:05 -0500
From:   Felix Kuehling <felix.kuehling@....com>
To:     Matthew Wilcox <willy@...radead.org>,
        Alex Sierra <alex.sierra@....com>
Cc:     jgg@...dia.com, david@...hat.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, akpm@...ux-foundation.org
Subject: Re: [PATCH v1 1/3] mm: split vm_normal_pages for LRU and non-LRU
 handling


Am 2022-03-10 um 14:25 schrieb Matthew Wilcox:
> On Thu, Mar 10, 2022 at 11:26:31AM -0600, Alex Sierra wrote:
>> @@ -606,7 +606,7 @@ static void print_bad_pte(struct vm_area_struct *vma, unsigned long addr,
>>    * PFNMAP mappings in order to support COWable mappings.
>>    *
>>    */
>> -struct page *vm_normal_page(struct vm_area_struct *vma, unsigned long addr,
>> +struct page *vm_normal_any_page(struct vm_area_struct *vma, unsigned long addr,
>>   			    pte_t pte)
>>   {
>>   	unsigned long pfn = pte_pfn(pte);
>> @@ -620,8 +620,6 @@ struct page *vm_normal_page(struct vm_area_struct *vma, unsigned long addr,
>>   			return NULL;
>>   		if (is_zero_pfn(pfn))
>>   			return NULL;
>> -		if (pte_devmap(pte))
>> -			return NULL;
>>   
>>   		print_bad_pte(vma, addr, pte, NULL);
>>   		return NULL;
> ... what?
>
> Haven't you just made it so that a devmap page always prints a bad PTE
> message, and then returns NULL anyway?

Yeah, that was stupid. :/  I think the long-term goal was to get rid of 
pte_devmap. But for now, as long as we have pte_special with pte_devmap, 
we'll need a special case to handle that like a normal page.

I only see the PFN_DEV|PFN_MAP flags set in a few places: 
drivers/dax/device.c, drivers/nvdimm/pmem.c, fs/fuse/virtio_fs.c. I 
guess we need to test at least one of them for this patch series to make 
sure we're not breaking them.


>
> Surely this should be:
>
> 		if (pte_devmap(pte))
> -			return NULL;
> +			return pfn_to_page(pfn);
>
> or maybe
>
> +			goto check_pfn;
>
> But I don't know about that highest_memmap_pfn check.

Looks to me like it should work. highest_memmap_pfn gets updated in 
memremap_pages -> pagemap_range -> move_pfn_range_to_zone -> 
memmap_init_range.

Regards,
   Felix


>
>> @@ -661,6 +659,22 @@ struct page *vm_normal_page(struct vm_area_struct *vma, unsigned long addr,
>>   	return pfn_to_page(pfn);
>>   }
>>   
>> +/*
>> + * vm_normal_lru_page -- This function gets the "struct page" associated
>> + * with a pte only for page cache and anon page. These pages are LRU handled.
>> + */
>> +struct page *vm_normal_lru_page(struct vm_area_struct *vma, unsigned long addr,
>> +			    pte_t pte)
> It seems a shame to add a new function without proper kernel-doc.
>