On 10/18/22 10:17 PM, Michał Mirosław wrote:
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?
Correct, max_pages with this comment is what I want. I'll update.
vec_len represents the total number of the page_range array elements. If
the pages which we want to return are sparse or the consective pages
have different flags, we'll only return one page in one page_range
struct. In this case if someone has specified max_pages to be 10,
vec_len must be at least 10 to keep store the 10 pages. So max_pages <=
vec_len.
[...]
/* 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?
Will do.
Best Regards
Michał Mirosław
