linux-kernel - Re: [RFC PATCH 0/3] Introduce new huge_ptep_get_access

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Message-ID: <97bb8f7e-38ce-4a21-fb76-4bd040ec00b7@linux.alibaba.com>
Date:   Mon, 9 May 2022 09:34:51 +0800
From:   Baolin Wang <baolin.wang@...ux.alibaba.com>
To:     Muchun Song <songmuchun@...edance.com>
Cc:     catalin.marinas@....com, will@...nel.org, arnd@...db.de,
        mike.kravetz@...cle.com, akpm@...ux-foundation.org, sj@...nel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-arch@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [RFC PATCH 0/3] Introduce new huge_ptep_get_access_flags()
 interface



On 5/8/2022 11:26 PM, Muchun Song wrote:
> On Sun, May 08, 2022 at 04:58:51PM +0800, Baolin Wang wrote:
>> Hi,
>>
>> As Mike pointed out [1], the huge_ptep_get() will only return one specific
>> pte value for the CONT-PTE or CONT-PMD size hugetlb on ARM64 system, which
>> will not take into account the subpages' dirty or young bits of a CONT-PTE/PMD
>> size hugetlb page. That will make us miss dirty or young flags of a CONT-PTE/PMD
>> size hugetlb page for those functions that want to check the dirty or
>> young flags of a hugetlb page. For example, the gather_hugetlb_stats() will
>> get inaccurate dirty hugetlb page statistics, and the DAMON for hugetlb monitoring
>> will also get inaccurate access statistics.
>>
>> To fix this issue, one approach is that we can define an ARM64 specific huge_ptep_get()
>> implementation, which will take into account any subpages' dirty or young bits.
> 
> IIUC, we could get the page size by page_size(pte_page(pte)).
> So, how about the following implementation of huge_ptep_get()?
> Does this work for you?
> 
> pte_t huge_ptep_get(pte_t *ptep)
> {
> 	int ncontig, i;
> 	size_t pgsize;
> 	pte_t orig_pte = ptep_get(ptep);
> 
> 	if (!pte_present(orig_pte) || !pte_cont(orig_pte))
> 		return orig_pte;
> 
> 	ncontig = num_contig_ptes(page_size(pte_page(orig_pte)), &pgsize);
> 
> 	for (i = 0; i < ncontig; i++, ptep++) {
> 		pte_t pte = ptep_get(ptep);
> 
> 		if (pte_dirty(pte))
> 			orig_pte = pte_mkdirty(orig_pte);
> 
> 		if (pte_young(pte))
> 			orig_pte = pte_mkyoung(orig_pte);
> 	}
> 
> 	return orig_pte;
> }

Thanks for your suggestion, and I think this works for me and looks more 
straight forward in case some functions using huge_ptep_get() will care 
about the young or dirty bits in future.

My only concern is that all the functions using huge_ptep_get() will set 
a contPTE dirty or accessed bit, however most functions do not care 
about the dirty and accessed bit, which becomes a bit more expensive for 
them? Also mentioned by Matthew in his comments. Anyway, I still think 
your suggestion is straight forward and I can change in next version if 
no other objections.