Quoting Douglas Anderson (2021-11-10 11:33:42)
> In commit 510410bfc034 ("drm/msm: Implement mmap as GEM object
> function") we switched to a new/cleaner method of doing things. That's
> good, but we missed a little bit.
>
> Before that commit, we used to _first_ run through the
> drm_gem_mmap_obj() case where `obj->funcs->mmap()` was NULL. That meant
> that we ran:
>
> vma->vm_flags |= VM_IO | VM_PFNMAP | VM_DONTEXPAND | VM_DONTDUMP;
> vma->vm_page_prot = pgprot_writecombine(vm_get_page_prot(vma->vm_flags));
> vma->vm_page_prot = pgprot_decrypted(vma->vm_page_prot);
>
> ...and _then_ we modified those mappings with our own. Now that
> `obj->funcs->mmap()` is no longer NULL we don't run the default
> code. It looks like the fact that the vm_flags got VM_IO / VM_DONTDUMP
> was important because we're now getting crashes on Chromebooks that
> use ARC++ while logging out. Specifically a crash that looks like this
> (this is on a 5.10 kernel w/ relevant backports but also seen on a
> 5.15 kernel):
>
> Unable to handle kernel paging request at virtual address ffffffc008000000
> Mem abort info:
> ESR = 0x96000006
> EC = 0x25: DABT (current EL), IL = 32 bits
> SET = 0, FnV = 0
> EA = 0, S1PTW = 0
> Data abort info:
> ISV = 0, ISS = 0x00000006
> CM = 0, WnR = 0
> swapper pgtable: 4k pages, 39-bit VAs, pgdp=000000008293d000
> [ffffffc008000000] pgd=00000001002b3003, p4d=00000001002b3003,
> pud=00000001002b3003, pmd=0000000000000000
> Internal error: Oops: 96000006 [#1] PREEMPT SMP
> [...]
> CPU: 7 PID: 15734 Comm: crash_dump64 Tainted: G W 5.10.67 #1 [...]
> Hardware name: Qualcomm Technologies, Inc. sc7280 IDP SKU2 platform (DT)
> pstate: 80400009 (Nzcv daif +PAN -UAO -TCO BTYPE=--)
> pc : __arch_copy_to_user+0xc0/0x30c
> lr : copyout+0xac/0x14c
> [...]
> Call trace:
> __arch_copy_to_user+0xc0/0x30c
> copy_page_to_iter+0x1a0/0x294
> process_vm_rw_core+0x240/0x408
> process_vm_rw+0x110/0x16c
> __arm64_sys_process_vm_readv+0x30/0x3c
> el0_svc_common+0xf8/0x250
> do_el0_svc+0x30/0x80
> el0_svc+0x10/0x1c
> el0_sync_handler+0x78/0x108
> el0_sync+0x184/0x1c0
> Code: f8408423 f80008c3 910020c6 36100082 (b8404423)
>
> Let's add the two flags back in.
>
> While we're at it, the fact that we aren't running the default means
> that we _don't_ need to clear out VM_PFNMAP, so remove that and save
> an instruction.
>
> NOTE: it was confirmed that VM_IO was the important flag to fix the
> problem I was seeing, but adding back VM_DONTDUMP seems like a sane
> thing to do so I'm doing that too.
>
> Fixes: 510410bfc034 ("drm/msm: Implement mmap as GEM object function")
> Reported-by: Stephen Boyd <swboyd@xxxxxxxxxxxx>
> Signed-off-by: Douglas Anderson <dianders@xxxxxxxxxxxx>
> ---
Reviewed-by: Stephen Boyd <swboyd@xxxxxxxxxxxx>
Tested-by: Stephen Boyd <swboyd@xxxxxxxxxxxx>
