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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 1/14/25 4:05 AM, Lorenzo Stoakes wrote: > + Willy for the fs/weirdness elements of this. > > On Mon, Jan 13, 2025 at 02:30:33PM -0800, Yang Shi wrote: >> When creating private mapping for /dev/zero, the driver makes it an >> anonymous mapping by calling set_vma_anonymous(). But it just sets >> vm_ops to NULL, vm_file is still valid and vm_pgoff is also file offset. > Hm yikes. > >> This is a special case and the VMA doesn't look like either anonymous VMA >> or file VMA. It confused other kernel subsystem, for example, khugepaged [1]. >> >> It seems pointless to keep such special case. Making private /dev/zero >> mapping a full anonymous mapping doesn't change the semantic of >> /dev/zero either. > My concern is that ostensibly there _is_ a file right? Are we certain that by > not setting this we are not breaking something somewhere else? > > Are we not creating a sort of other type of 'non-such-beast' here? But the file is /dev/zero. I don't see this could break the semantic of /dev/zero. The shared mapping of /dev/zero is not affected by this change, kernel already treated private mapping of /dev/zero as anonymous mapping, but with some weird settings in VMA. When reading the mapping, it returns 0 with zero page, when writing the mapping, a new anonymous folio is allocated. > > I mean already setting it anon and setting vm_file non-NULL is really strange. > >> The user visible effect is the mapping entry shown in /proc//smaps >> and /proc//maps. >> >> Before the change: >> ffffb7190000-ffffb7590000 rw-p 00001000 00:06 8 /dev/zero >> >> After the change: >> ffffb6130000-ffffb6530000 rw-p 00000000 00:00 0 >> > Yeah this seems like it might break somebody to be honest, it's really > really really strange to map a file then for it not to be mapped. Yes, it is possible if someone really care whether the anonymous-like mapping is mapped by /dev/zero or just created by malloc(). But I don't know who really do... > > But it's possibly EVEN WEIRDER to map a file and for it to seem mapped as a > file but for it to be marked anonymous. > > God what a mess. > >> [1]: https://lore.kernel.org/linux-mm/20250111034511.2223353-1-liushixin2@huawei.com/ > I kind of hate that we have to mitigate like this for a case that should > never ever happen so I'm inclined towards your solution but a lot more > inclined towards us totally rethinking this. > > Do we _have_ to make this anonymous?? Why can't we just reference the zero > page as if it were in the page cache (Willy - feel free to correct naive > misapprehension here). TBH, I don't see why page cache has to be involved. When reading, 0 is returned by zero page. When writing a CoW is triggered if page cache is involved, but the content of the page cache should be just 0, so we copy 0 to the new folio then write to it. It doesn't make too much sense. I think this is why private /dev/zero mapping is treated as anonymous mapping in the first place. > >> Signed-off-by: Yang Shi >> --- >> drivers/char/mem.c | 4 ++++ >> 1 file changed, 4 insertions(+) >> >> diff --git a/drivers/char/mem.c b/drivers/char/mem.c >> index 169eed162a7f..dae113f7fc1b 100644 >> --- a/drivers/char/mem.c >> +++ b/drivers/char/mem.c >> @@ -527,6 +527,10 @@ static int mmap_zero(struct file *file, struct vm_area_struct *vma) >> if (vma->vm_flags & VM_SHARED) >> return shmem_zero_setup(vma); >> vma_set_anonymous(vma); >> + fput(vma->vm_file); >> + vma->vm_file = NULL; >> + vma->vm_pgoff = vma->vm_start >> PAGE_SHIFT; > Hmm, this might have been mremap()'d _potentially_ though? And then now > this will be wrong? But then we'd have no way of tracking it correctly... I'm not quite familiar with the subtle details and corner cases of meremap(). But mmap_zero() should be called by mmap(), so the VMA has not been visible to user yet at this point IIUC. How come mremap() could move it? > > I've not checked the function but do we mark this as a special mapping of > some kind? > >> + >> return 0; >> } >> >> -- >> 2.47.0 >>