Re: [PATCH v5 6/7] selftests: mm: Add a test for remapping within a range

[Lorenzo Stoakes <lstoakes@gmail.com>]

On Mon, Aug 28, 2023 at 06:59:12PM +0000, Joel Fernandes wrote:
> On Sun, Aug 27, 2023 at 11:15:20AM +0100, Lorenzo Stoakes wrote:
> > On Sun, Aug 27, 2023 at 10:57:59AM +0100, Lorenzo Stoakes wrote:
> > [snip]
> >
> > > > +/*
> > > > + * Verify that an mremap within a range does not cause corruption
> > > > + * of unrelated part of range.
> > > > + *
> > > > + * Consider the following range which is 2MB aligned and is
> > > > + * a part of a larger 10MB range which is not shown. Each
> > > > + * character is 256KB below making the source and destination
> >
> > Just noticed, I think you misspeak here, as this test doens't seem to
> > offset by 256 KiB? That is the strategy for mremap_move_1mb_from_start()
> > rather than this test so perhaps comment needs to be moved around?
> >
> >  * 2MB each. The lower case letters are moved (s to d) and the
> >  * upper case letters are not moved. The below test verifies
> >  * that the upper case S letters are not corrupted by the
> >  * adjacent mremap.
> >  *
> >  * |DDDDddddSSSSssss|
> >  */
> >  static void mremap_move_within_range(char pattern_seed)
>
> Here we are moving 1MB within a 4MB zone of a large mapping. Each character
> 's' or 'd' is 256KB. The 256KB there is just for illustration and not really
> significant as such. The 'ssss' is moved to 'dddd' 1MB each. Here we make

Ahhh I see. I find that a little confusing here, perhaps clearer to say 'each
block of letters is 1 MiB in size' or something?

> sure that this move did not accidentally corrupt 'SSSS' and 'DDDD' due to
> alignment optimization. Basically to protect from this, we check in the code
> that the source address is beginning of the VMA:
> +	if (vma->vm_start != addr_to_align)
> +		return false;
>
> But you did point an issue which is I need to change the comment from 'larger
> 10MB' to 'larger 20MB'.

Did I? Well happy to take credit ;)

>
> In the mremap_move_1mb_from_start() test, I request for an alignment of
> 1.25MB so that when I align down, I fall no mapping. This is to catch a bug
> that Linus found which is that just because an aligned down address did not
> fall on a mapping, that doesn't mean we can just move it at PMD-level
> otherwise we destroy the mapping.

Yeah that case it all makes sense in, just wondered if the comment belonged
there but just me not reading your comment carefully!

>
> I do need to update the test name in mremap_move_1mb_from_start() to: "mremap
> move 1mb from start at 1MB+256KB aligned src". So thanks for point this!
>
> Would that sort it out or is there still something in the comment I am
> missing?

Just the suggestion above re: clarification, however it's not a show
stopper more of a small nit.

>
> Thanks!
>
>  - Joel

Cheers!

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> > [snip]