linux-kernel - Re: [PATCH -v7 2/2] Update ctime and mtime for memory-mapped files

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Date:	Mon, 21 Jan 2008 18:16:46 -0800 (PST)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Anton Salikhmetov <salikhmetov@...il.com>
cc:	linux-mm@...ck.org, jakob@...hought.net,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	valdis.kletnieks@...edu, riel@...hat.com, ksm@...dk,
	staubach@...hat.com, jesper.juhl@...il.com, a.p.zijlstra@...llo.nl,
	Andrew Morton <akpm@...ux-foundation.org>, protasnb@...il.com,
	miklos@...redi.hu, r.e.wolff@...wizard.nl,
	hidave.darkstar@...il.com, hch@...radead.org
Subject: Re: [PATCH -v7 2/2] Update ctime and mtime for memory-mapped files

On Tue, 22 Jan 2008, Anton Salikhmetov wrote:
>  
>  /*
> + * Scan the PTEs for pages belonging to the VMA and mark them read-only.
> + * It will force a pagefault on the next write access.
> + */
> +static void vma_wrprotect(struct vm_area_struct *vma)
> +{
> +	unsigned long addr;
> +
> +	for (addr = vma->vm_start; addr < vma->vm_end; addr += PAGE_SIZE) {
> +		spinlock_t *ptl;
> +		pgd_t *pgd = pgd_offset(vma->vm_mm, addr);
> +		pud_t *pud = pud_offset(pgd, addr);
> +		pmd_t *pmd = pmd_offset(pud, addr);
> +		pte_t *pte = pte_offset_map_lock(vma->vm_mm, pmd, addr, &ptl);

This is extremely expensive over bigger areas, especially sparsely mapped 
ones (it does all the lookups for all four levels over and over and over 
again for eachg page).

I think Peter Zijlstra posted a version that uses the regular kind of 
nested loop (with inline functions to keep the thing nice and clean), 
which gets rid of that.

[ The sad/funny part is that this is all how we *used* to do msync(), back 
  in the days: we're literally going back to the "pre-cleanup" logic. See 
  commit 204ec841fbea3e5138168edbc3a76d46747cc987: "mm: msync() cleanup" 
  for details ]

Quite frankly, I really think you might be better off just doing a

	git revert 204ec841fbea3e5138168edbc3a76d46747cc987

and working from there! I just checked, and it still reverts cleanly, and 
you'd end up with a nice code-base that (a) has gotten years of testing 
and (b) already has the looping-over-the-pagetables code.

			Linus
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