I mean we should be able to specify for what pages we need to get info
for. An ioctl argument can have these four fields:
* required bits (rmask & mask == mask) - all bits from this mask have to be set.
* any of these bits (amask & mask != 0) - any of these bits is set.
* exclude masks (emask & mask == 0) = none of these bits are set.
* return mask - bits that have to be reported to user.
The required mask (rmask) makes sense to me. At the moment, I only know
about the practical use case for the required mask. Can you share how
can any and exclude masks help for the CRIU?
I looked at should_dump_page in the CRIU code:
https://github.com/checkpoint-restore/criu/blob/45641ab26d7bb78706a6215fdef8f9133abf8d10/criu/mem.c#L102
When CRIU dumps file private mappings, it needs to get pages that have
PME_PRESENT or PME_SWAP but don't have PME_FILE.
I would really like to see the mask discussed will be adopted. With it CRIU will
be able to migrate huge sparse VMAs assuming that a single hole is processed in
O(1) time.
Use cases for migrating sparse VMAs are binaries sanitized with ASAN, MSAN or
TSAN [1]. All of these sanitizers produce sparse mappings of shadow memory [2].
Being able to migrate such binaries allows to highly reduce the amount of work
needed to identify and fix post-migration crashes, which happen constantly.
Hello all,
I've included the masks which the CRIU developers have specified.
max_out_page is another new optional variable which is needed to
terminate the operation without visiting all the pages after finding the
max_out_page number of desired pages. There is no way to terminate the
operation without this variable.
How does the interface looks now? Please comment.
/* PAGEMAP IOCTL */
#define PAGEMAP_GET _IOWR('f', 16, struct pagemap_sd_args)
#define PAGEMAP_CLEAR _IOWR('f', 17, struct pagemap_sd_args)
#define PAGEMAP_GET_AND_CLEAR _IOWR('f', 18, struct pagemap_sd_args)
/* Bits are set in the bitmap of the page_region and masks in
pagemap_sd_args */
#define PAGE_IS_SD 1 << 0
#define PAGE_IS_FILE 1 << 1
#define PAGE_IS_PRESENT 1 << 2
#define PAGE_IS_SWAPED 1 << 3
/**
* struct page_region - Page region with bitmap flags
* @start: Start of the region
* @len: Length of the region
* bitmap: Bits sets for the region
*/
struct page_region {
__u64 start;
__u64 len;
__u64 bitmap;
};
/**
* struct pagemap_sd_args - Soft-dirty IOCTL argument
* @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)
* @flags: Special flags for the IOCTL
* @rmask: Special flags for the IOCTL
* @amask: Special flags for the IOCTL
* @emask: Special flags for the IOCTL
* @__reserved: Reserved member to preserve data alignment. Must be 0.
*/
struct pagemap_sd_args {
__u64 __user start;
__u64 len;
__u64 __user vec; // page_region
__u64 vec_len; // sizeof(page_region)
__u32 flags; // special flags
__u32 rmask;
__u32 amask;
__u32 emask;
__u32 max_out_page;
__u32 __reserved;
};
/* Special flags */
#define PAGEMAP_NO_REUSED_REGIONS 0x1
- Clear the pages which are soft-dirty.
- The optional flag to ignore the VM_SOFTDIRTY and only track per page
soft-dirty PTE bit
There are two decisions which have been taken about how to get the output
from the syscall.
- Return offsets of the pages from the start in the vec
We can conside to return regions that contains pages with the same set
of bits.
struct page_region {
void *start;
long size;
u64 bitmap;
}
And ioctl returns arrays of page_region-s. I believe it will be more
compact form for many cases.
Thank you for mentioning this. I'd considered this while development.
But I gave up and used the simple array to return the offsets of the
pages as in the problem I'm trying to solve, the dirty pages may be
present amid non-dirty pages. The range may not be useful in that case.
This is a good example. If we expect more than two consequent pages
on average, the "region" interface looks more prefered. I don't know your
use-case, but in the case of CRIU, this assumption looks reasonable.
Plus one for page_region data structure. It will make ASAN shadow memory
representation much more compact as well as any other classical use-case.
[1] https://github.com/google/sanitizers
[2] https://github.com/google/sanitizers/wiki/AddressSanitizerAlgorithm#64-bit
Best,
Danylo
