lkml.org 
[lkml]   [2022]   [Sep]   [23]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: [PATCH v10 1/8] mm: introduce FOLL_PCI_P2PDMA to gate getting PCI P2PDMA pages


    On 2022-09-23 17:21, Jason Gunthorpe wrote:
    > On Fri, Sep 23, 2022 at 05:14:11PM -0600, Logan Gunthorpe wrote:
    >>
    >>
    >> On 2022-09-23 17:07, Jason Gunthorpe wrote:
    >>> On Fri, Sep 23, 2022 at 05:01:26PM -0600, Logan Gunthorpe wrote:
    >>>>
    >>>>
    >>>>
    >>>> On 2022-09-23 16:58, Jason Gunthorpe wrote:
    >>>>> On Fri, Sep 23, 2022 at 02:11:03PM -0600, Logan Gunthorpe wrote:
    >>>>>>
    >>>>>>
    >>>>>> On 2022-09-23 13:53, Jason Gunthorpe wrote:
    >>>>>>> On Fri, Sep 23, 2022 at 01:08:31PM -0600, Logan Gunthorpe wrote:
    >>>>>>> I'm encouraging Dan to work on better infrastructure in pgmap core
    >>>>>>> because every pgmap implementation has this issue currently.
    >>>>>>>
    >>>>>>> For that reason it is probably not so relavent to this series.
    >>>>>>>
    >>>>>>> Perhaps just clarify in the commit message that the FOLL_LONGTERM
    >>>>>>> restriction is to copy DAX until the pgmap page refcounts are fixed.
    >>>>>>
    >>>>>> Ok, I'll add that note.
    >>>>>>
    >>>>>> Per the fix for the try_grab_page(), to me it doesn't fit well in
    >>>>>> try_grab_page() without doing a bunch of cleanup to change the
    >>>>>> error handling, and the same would have to be added to try_grab_folio().
    >>>>>> So I think it's better to leave it where it was, but move it below the
    >>>>>> respective grab calls. Does the incremental patch below look correct?
    >>>>>
    >>>>> Oh? I was thinking of just a very simple thing:
    >>>>
    >>>> Really would like it to return -EREMOTEIO instead of -ENOMEM as that's the
    >>>> error used for bad P2PDMA page everywhere.
    >>>
    >>> I'd rather not see GUP made more fragile just for that..
    >>
    >> Not sure how that's more fragile... You're way seems more dangerous given
    >> the large number of call sites we are adding it to when it might not
    >> apply.
    >
    > No, that is the point, it *always* applies. A devmap struct page of
    > the wrong type should never exit gup, from any path, no matter what.
    >
    > We have two central functions that validate a page is OK to return,
    > that *everyone* must call.
    >
    > If you don't put it there then we will probably miss copying it into a
    > call site eventually.

    Most of the call sites don't apply though, with huge pages and gate pages...

    >>> try_grab_page() calls folio_ref_inc(), that is only legal if it knows
    >>> the page is already a valid pointer under the PTLs, so it is safe to
    >>> check the pgmap as well.
    >>
    >> My point is it doesn't get a reference or a pin unless FOLL_PIN or FOLL_GET is
    >> set and the documentation states that neither might be set, in which case
    >> folio_ref_inc() will not be called...
    >
    > That isn't how GUP is structured, all the calls to try_grab_page() are
    > in places where PIN/GET might be set and are safe for that usage.
    >
    > If we know PIN/GET is not set then we don't even need to call the
    > function because it is a NOP.

    That's not what the documentation for the function says:

    "Either FOLL_PIN or FOLL_GET (or neither) may be set... Return: true for success,
    or if no action was required (if neither FOLL_PIN nor FOLL_GET was set, nothing
    is done)."

    https://elixir.bootlin.com/linux/v6.0-rc6/source/mm/gup.c#L194

    Logan

    \
     
     \ /
      Last update: 2022-09-24 01:36    [W:3.409 / U:0.832 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site