On Wed, Sep 04, 2019 at 07:00:56PM +0200, Dominique Martinet wrote: > Dominique Martinet wrote on Tue, Sep 03, 2019: > > Matthew Wilcox wrote on Tue, Sep 03, 2019: > > > > What I'd like to know is: > > > > - we know (assuming the other side isn't too bugged, but if it is we're > > > > fucked up anyway) exactly what huge-page-sized physical memory range has > > > > been mapped on the other side, is there a way to manually gather the > > > > pages corresponding and merge them into a huge page? > > > > > > You're using the word 'page' here, but I suspect what you really mean is > > > "pfn" or "pte". As you've described it, it doesn't matter what data structure > > > Linux is using for the memory, since Linux doesn't know about the memory. > > > > Correct, we're already using vmf_insert_pfn > > Actually let me take that back, vmf_insert_pfn is only used if > pfn_valid() is false, probably as a safeguard of sort(?). > The normal case went with pfn_to_page(pfn) + vm_insert_page() so, as > things stands. > I do have a few more questions if you could humor me a bit more... > > - the vma was created with a vm_flags including VM_MIXEDMAP for some > reason, I don't know why. > If I change it to VM_PFNMAP (which sounds better here from the little I > understand of this as we do not need cow and looks a bit simpler?), I > can remove the vm_insert_page() path and use the vmf_insert_pfn one > instead, which appears to work fine for simple programs... But the > kernel thread for my network adapter (bxi... which is not upstream > either I guess.. sigh..) no longer tries to fault via my custom .fault > vm operation... Which means I probably did need MIXEDMAP ? Strange ... PFNMAP absolutely should try to fault via the ->fault vm operation (although see below) > - ignoring that for now (it's not like I need to switch to PFNMAP); > adding vmf_insert_pfn_pmd() for when the remote side uses large pages, > it complains that the vmf->pmd is not a pmd_none nor huge nor a devmap > (this check appears specific to rhel7 kernel, I could temporarily test > with an upstream kernel but the network adapter won't work there so I'll > need this to work on this ultimately) > > It looks like handle_mm_fault() will always try to allocate a pmd so it > should never be empty in my fault handler, and I don't see anything else > than vmf_insert_pfn_pmd() setting the mkdevmap flag, and it's not huge > either... > (on a dump, the the pmd content is 175cb18067, so these flags according > to crash for x86_64 are (PRESENT|RW|USER|ACCESSED|DIRTY)) > > I tried adding a huge_fault vm op thinking it might be called with a > more appropriate pmd but it doesn't seem to be called at all in my > case..? I would have assumed from the code that it would try every page You shouldn't be calling vmf_insert_pfn_pmd() from a regular ->fault handler, as by then the fault handler has already inserted a PMD. The ->huge_fault handler is the place to call it from. You may need to force PMD-alignment for your call to mmap(). > Long story short, I think I have some deeper undestanding problem about > the whole thing. Do I also need to use some specific flags when that > special file is mmap'd to allow huge_fault to be called ? > I think transparent_hugepage_enabled(vma) is fine, but the vmf.pmd found > in __handle_mm_fault is probably already not none at this point...? > > Thanks again, feel free to ignore me for a bit longer I'll keep digging > my own grave, writing to a rubber duck that might have an idea of how > far the wrong way I've gone already helps... :D Hope these pointers are slightly more useful than a rubber duck ;-)
