On Mon, Oct 03, 2022 at 04:21:22PM +0500, Muhammad Usama Anjum wrote: > On 9/28/22 10:24 PM, Andrei Vagin wrote: > > On Wed, Sep 21, 2022 at 11:26 AM Muhammad Usama Anjum > > <usama.anjum@xxxxxxxxxxxxx> wrote: > >> > >> Hi, > >> > >> Thank you for reviewing. > >> > >> On 9/19/22 7:58 PM, Andrei Vagin wrote: > >>>> This ioctl can be used by the CRIU project and other applications which > >>>> require soft-dirty PTE bit information. The following operations are > >>>> supported in this ioctl: > >>>> - Get the pages that are soft-dirty. > >>> > >>> I think this interface doesn't have to be limited by the soft-dirty > >>> bits only. For example, CRIU needs to know whether file, present and swap bits > >>> are set or not. > >> These operations can be performed by pagemap procfs file. Definitely > >> performing them through IOCTL will be faster. But I'm trying to add a > >> simple IOCTL by which some specific PTE bit can be read and cleared > >> atomically. This IOCTL can be extended to include other bits like file, > >> present and swap bits by keeping the interface simple. The following > >> mask advice is nice. But if we add that kind of masking, it'll start to > >> look like a filter on top of pagemap. My intention is to not duplicate > >> the functionality already provided by the pagemap. One may ask, then why > >> am I adding "get the soft-dirty pages" functionality? I'm adding it to > >> complement the get and clear operation. The "get" and "get and clear" > >> operations with special flag (PAGEMAP_SD_NO_REUSED_REGIONS) can give > >> results quicker by not splitting the VMAs. > > > > This simple interface is good only for a limited number of use-cases. > > The interface > > that I suggest doesn't duplicate more code than this one, but it is much more > > universal. It will be a big mess if you add a separate API for each > > specific use-case. > > > > > >>> 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. > >>>> - 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. > > > >> Also we want to return only a specific number of pages of interest. The > >> following paragraph explains it. > >> > >>> > >>>> - Stop execution when vec is filled with dirty pages > >>>> These two arguments doesn't follow the mincore() philosophy where the > >>>> output array corresponds to the address range in one to one fashion, hence > >>>> the output buffer length isn't passed and only a flag is set if the page > >>>> is present. This makes mincore() easy to use with less control. We are > >>>> passing the size of the output array and putting return data consecutively > >>>> which is offset of dirty pages from the start. The user can convert these > >>>> offsets back into the dirty page addresses easily. Suppose, the user want > >>>> to get first 10 dirty pages from a total memory of 100 pages. He'll > >>>> allocate output buffer of size 10 and the ioctl will abort after finding the > >>>> 10 pages. This behaviour is needed to support Windows' getWriteWatch(). The > >>>> behaviour like mincore() can be achieved by passing output buffer of 100 > >>>> size. This interface can be used for any desired behaviour. > > > > Now, it is more clear where this interface came from. It repeats the > > interface of Windows' getWriteWatch. I think we have to look wider. > > The interface that reports regions will be more efficient for many > > use-cases. As for getWriteWatch, it will require a bit more code in > > user-space, but this code is trivial. I added Danylo to CC. I think he has a good use-case for the new interface. Danylo, could you describe it here. > > > > Thanks, > > Andrei > > -- > Muhammad Usama Anjum
