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charset=utf-8 Content-Transfer-Encoding: quoted-printable X-Stat-Signature: gen695m3uwkzk86ifju8wx4j81ooqrdu X-Rspamd-Queue-Id: D2DD510000F X-Rspamd-Server: rspam06 X-Rspam-User: X-HE-Tag: 1713554678-468736 X-HE-Meta: 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 kHor1ZiC m0UNH9w1+0rfUq72KZcKuuFaFHB6PXnB1u8CB+ovVKitKsEDx0w38aWW1mZvuqcZis0230gpP3eLpHoIrF6elyPt+xXkXUpvtUD/hyn7VGbAkw3vG2Gpo/o6wMebBUZuuAppRqRCeKy8hV44Hs+w0LtiBi9IZPasrW8ay6UAYDhatN9QVwoVMwg5n+tNzmhlyXqsktT4/Q69IJm8vH+YGKeY5Vuewke/+zcL8d3jQHqAAdbnnkCmvYKaHVXUnkwPNeMqMVfjS6jwzclfB43wr0Aj/nXDFR/vuyMMSsmB8oEYrnj0IxIS5Dmi2A6T/yU/w8loo 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 19.04.24 19:23, Boqun Feng wrote: > On Fri, Apr 19, 2024 at 08:36:11AM +0000, Benno Lossin wrote: >> On 19.04.24 01:04, Boqun Feng wrote: >>> On Thu, Apr 18, 2024 at 03:56:11PM -0700, Boqun Feng wrote: >>>> On Thu, Apr 18, 2024 at 10:08:40PM +0000, Benno Lossin wrote: >>>>> On 18.04.24 20:52, Boqun Feng wrote: >>>>>> On Thu, Apr 18, 2024 at 08:59:20AM +0000, Alice Ryhl wrote: >>>>>>> + /// Runs a piece of code with a raw pointer to a slice of this= page, with bounds checking. >>>>>>> + /// >>>>>>> + /// If `f` is called, then it will be called with a pointer th= at points at `off` bytes into the >>>>>>> + /// page, and the pointer will be valid for at least `len` byt= es. The pointer is only valid on >>>>>>> + /// this task, as this method uses a local mapping. >>>>>>> + /// >>>>>>> + /// If `off` and `len` refers to a region outside of this page= , then this method returns >>>>>>> + /// `EINVAL` and does not call `f`. >>>>>>> + /// >>>>>>> + /// # Using the raw pointer >>>>>>> + /// >>>>>>> + /// It is up to the caller to use the provided raw pointer cor= rectly. The pointer is valid for >>>>>>> + /// `len` bytes and for the duration in which the closure is c= alled. The pointer might only be >>>>>>> + /// mapped on the current thread, and when that is the case, d= ereferencing it on other threads >>>>>>> + /// is UB. Other than that, the usual rules for dereferencing = a raw pointer apply: don't cause >>>>>>> + /// data races, the memory may be uninitialized, and so on. >>>>>>> + /// >>>>>>> + /// If multiple threads map the same page at the same time, th= en they may reference with >>>>>>> + /// different addresses. However, even if the addresses are di= fferent, the underlying memory is >>>>>>> + /// still the same for these purposes (e.g., it's still a data= race if they both write to the >>>>>>> + /// same underlying byte at the same time). >>>>>>> + fn with_pointer_into_page( >>>>>>> + &self, >>>>>>> + off: usize, >>>>>>> + len: usize, >>>>>>> + f: impl FnOnce(*mut u8) -> Result, >>>>>> >>>>>> I wonder whether the way to go here is making this function signatur= e: >>>>>> >>>>>> fn with_slice_in_page ( >>>>>> &self, >>>>>> =09 off: usize, >>>>>> =09 len: usize, >>>>>> =09 f: iml FnOnce(&UnsafeCell<[u8]>) -> Result >>>>>> ) -> Result >>>>>> >>>>>> , because in this way, it makes a bit more clear that what memory th= at >>>>>> `f` can access, in other words, the users are less likely to use the >>>>>> pointer in a wrong way. >>>>>> >>>>>> But that depends on whether `&UnsafeCell<[u8]>` is the correct >>>>>> abstraction and the ecosystem around it: for example, I feel like th= ese >>>>>> two functions: >>>>>> >>>>>> =09 fn len(slice: &UnsafeCell<[u8]>) -> usize >>>>>> =09 fn as_ptr(slice: &UnsafeCell<[u8]>) -> *mut u8 >>>>>> >>>>>> should be trivially safe, but I might be wrong. Again this is just f= or >>>>>> future discussion. >>>>> >>>>> I think the "better" type would be `&[UnsafeCell]`. Since there y= ou >>>>> can always access the length. >>>>> >>>> >>>> Hmm.. here is the thing, having `&UnsafeCell<[u8]>` means having a `*m= ut >>>> [u8]>`, and it should always be safe to get a "length" of `*mut [u8]`, >>>> right? I haven't found any method doing that, but the length should be >>>> just a part of fat pointer, so I think getting that is a defined >>>> behavior. But maybe I'm missing something. >> >> There is `to_raw_parts` [1], but that is unstable. (Note that >> `<[T] as Pointee>::Metadata =3D usize`, see [2]) >> >=20 > Oh, that's good to know, thank you! ;-) > >>> Hmm... but I guess one of the problems of this approach, is how to >>> construct a `&UnsafeCell<[u8]>` from a pointer and length... >> >> We could use `from_raw_parts` [3]. But when making the slice the outer >> type, we can use a stable function to convert a pointer and a length to >> a slice [4]. >> >=20 > Yes, but there appears no way to get a pointer with larger provenance > from a `&[UnsafeCell]`, right? What do you mean by "larger provenance"? >>>>> Another question would be if page allows for uninitialized bits, in t= hat >>>>> case, we would need `&[Opaque]`. >>>>> >>>> >>>> Yes, or `&Opaque<[u8>]`. >> >> I don't think that putting the slice on the inside is what we want. Also >=20 > Hmm.. why? So in `&UnsafeCell<[u8]>` vs `&[UnsafeCell]` case, I > think the former represent "a slice of u8 that can be modified in the > same time" very well, and this is what a pointer-and-length pair usually > represents in kernel, I think. But yes, the latter is OK to me as well, > just hard to play the provenance game I guess? Ultimately it again comes down to missing field projections :) The type `&UnsafeCell<[u8]>` is less *useful*, since you cannot even get the length of the slice. Also indexing into this type is not easily possible. This is because the only way to get/change the inner value of an `UnsafeCell` is via `get`. Compare this with the slice type. It allows getting the length, indexing into it (ie a form of field projections, if we consider slices as having a variable amount of fields). All those issues would be solved by (good) field projections. Field projections also give a reason for why using `&[UnsafeCell]` is not really different from `&UnsafeCell<[u8]>`: At any point in time we ought to be able to project `&UnsafeCell<[u8]> -> &[UnsafeCell]`. So it's fine to just use that from the get-go. >> note that `Opaque` requires that `T: Sized` and that is not the case >> for `[u8]`. >=20 > Oh, you're right. In case of MaybeUninit, it requires `T: Sized`, so > `Opaque<[u8]>` doesn't quite work. >=20 > Moving forward, maybe the first step is to see whether `&[Opaque]` > and `&[UnsafeCell]` can have a good way to generate a pointer with > proper provenance? Time to ping t-opsem maybe? Good idea, do you want to do that, or should I do it? --=20 Cheers, Benno