linux-kernel - Re: [PATCH -next 3/7] mm: memory: convert do_cow

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Date:   Mon, 16 Jan 2023 19:31:43 +0800
From:   Kefeng Wang <wangkefeng.wang@...wei.com>
To:     Matthew Wilcox <willy@...radead.org>
CC:     <akpm@...ux-foundation.org>, <linux-mm@...ck.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH -next 3/7] mm: memory: convert do_cow_fault to use folios



On 2023/1/13 23:37, Matthew Wilcox wrote:
> On Thu, Jan 12, 2023 at 04:30:02PM +0800, Kefeng Wang wrote:
>> -	vmf->cow_page = alloc_page_vma(GFP_HIGHUSER_MOVABLE, vma, vmf->address);
>> -	if (!vmf->cow_page)
>> +	cow_folio = vma_alloc_folio(GFP_HIGHUSER_MOVABLE, 0, vma, vmf->address,
>> +				    false);
>> +	if (!cow_folio)
> 
> I have a patch I've been sitting on that converts vmf->cow_page to be
> a folio.  I think this series is well and truly wrecked at this point,
> so let me go back and dig it out; see if it still makes sense.
> 

For now, as vmf->page and vmf->cow_page used in do_cow_page(), it only 
supports cow on a page, and after converting, the folio still is 0 order,

> I'm a bit unsure about it because maybe we want to allocate
> high(ish)-order folios on COW fault, and if we do, then maybe we want
> to align them in some way with the virtual addresses, or the other
> folios in the VMA.  And then we might want to indicate the precise
> page for this page fault rather than have this page fault be the
> start of a multi-order folio.
> 
This means that if high(ish)-order folios/multi-order folio on cow, it
needs additional jobs and precise page for this pagefault, but for 
order-0, the converting is right but could break/mislead future logical? 
not very clear about this part, but let's wait for your patches.