Messages in this thread | | | Date | Tue, 18 Oct 2022 18:32:46 +0500 | Subject | Re: [PATCH v3 0/4] Implement IOCTL to get and clear soft dirty PTE | From | Muhammad Usama Anjum <> |
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On 10/18/22 3:36 PM, Muhammad Usama Anjum wrote: >>>>>>> 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. > Updated interface with only one IOCTL. If vec is defined, get operation will be performed. If PAGEMAP_SD_CLEAR flag is specified, soft dirty bit will be cleared as well. CLEAR flag can only be specified for clearing soft dirty bit.
/* PAGEMAP IOCTL */ #define PAGEMAP_SCAN _IOWR('f', 16, 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 of the page * @len: Length of the region (All the pages in this length are included) * @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: Required mask - All of these bits have to be set * @amask: Any mask - Any of these bits are set * @emask: Exclude mask - None of these bits are set * @rmask: Bits that have to be reported to the user in page_region */ struct pagemap_scan_args { __u64 __user start; __u64 len; __u64 __user vec; __u64 vec_len; __u32 max_out_page; __u32 flags; __u32 rmask; __u32 amask; __u32 emask; __u32 rmask; };
/* Special flags */ #define PAGEMAP_SD_CLEAR 1 << 0 #define PAGEMAP_NO_REUSED_REGIONS 1 << 1
> /* 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 >>
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