Re: [PATCH v3 0/4] Implement IOCTL to get and clear soft dirty PTE

[Michał Mirosław]

On Tue, 18 Oct 2022 at 15:23, Muhammad Usama Anjum
<usama.anjum@xxxxxxxxxxxxx> wrote:
>
> On 10/18/22 4:11 PM, Michał Mirosław wrote:
> > On Tue, 18 Oct 2022 at 12:36, Muhammad Usama Anjum
> > <usama.anjum@xxxxxxxxxxxxx> wrote:
[...]
> >>    * @start:             Starting address
> >>    * @len:               Length of the region
> >>    * @vec:               Output page_region struct array
> >>    * @vec_len:           Length of the page_region struct array
> >>    * @max_out_page:      Optional max output pages (It must be less than
> >> vec_len if specified)
> >
> > Why is it required to be less than vec_len? vec_len effectively
> > specifies max number of ranges to find, and this new additional field
> > counts pages, I suppose?
> > BTW, if we count pages, then what size of them? Maybe using bytes
> > (matching start/len fields) would be more consistent?
> Yes, it if for counting pages. As the regions can have multiple pages,
> user cannot specify through the number of regions that how many pages
> does he need. Page size is used here as well like the start and len.
> This is optional argument as this is only needed to emulate the Windows
> syscall getWriteWatch.

I'm wondering about the condition that max_out_page < vec_len. Since
both count different things (pages vs ranges) I would expect there is
no strict relation between them and information returned is as much as
fits both (IOW: at most vec_len ranges spanning not more than
max_out_page pages). The field's name and description I'd suggest
improving: maybe 'max_pages' with a comment that 0 = unlimited?

[...]
> >> /* Special flags */
> >> #define PAGEMAP_NO_REUSED_REGIONS       0x1
> >
> > What does this flag do?
> Some non-dirty pages get marked as dirty because of the kernel's
> internal activity. The dirty bit of the pages is stored in the VMA flags
> and in the per page flags. If any of these two bits are set, the page is
> considered to be dirty. Suppose you have cleared the dirty bit of half
> of VMA which will be done by splitting the VMA and clearing dirty flag
> in the half VMA and the pages in it. Now kernel may decide to merge the
> VMAs again as dirty bit of VMAs isn't considered if the VMAs should be
> merged. So the half VMA becomes dirty again. This splitting/merging
> costs performance. The application receives a lot of pages which aren't
> dirty in reality but marked as dirty. Performance is lost again here.
>
> This PAGEMAP_NO_REUSED_REGIONS flag is used to don't depend on the dirty
> flag in the VMA flags. It only depends on the individual page dirty bit.
> With doing this, the new memory regions which are just created, doesn't
> look like dirty when seen with the IOCTL, but look dirty when seen from
> pagemap. This seems okay as the user of this flag know the implication
> of using it.

Thanks for explaining! Could you include this as a comment in the patch?

Best Regards
Michał Mirosław