linux-kernel - Re: [PATCH v3 10/17] arm64, trans_pgd: adjust trans_pgd_create

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Message-ID: <CA+CK2bAS37vPa0FD7Ya1vnZR29hiEsNfkq6q7+UreNRjRgUEFw@mail.gmail.com>
Date:   Fri, 6 Sep 2019 15:03:42 -0400
From:   Pavel Tatashin <pasha.tatashin@...een.com>
To:     James Morse <james.morse@....com>
Cc:     James Morris <jmorris@...ei.org>, Sasha Levin <sashal@...nel.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        kexec mailing list <kexec@...ts.infradead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Jonathan Corbet <corbet@....net>,
        Catalin Marinas <catalin.marinas@....com>, will@...nel.org,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Marc Zyngier <marc.zyngier@....com>,
        Vladimir Murzin <vladimir.murzin@....com>,
        Matthias Brugger <matthias.bgg@...il.com>,
        Bhupesh Sharma <bhsharma@...hat.com>,
        linux-mm <linux-mm@...ck.org>,
        Mark Rutland <mark.rutland@....com>
Subject: Re: [PATCH v3 10/17] arm64, trans_pgd: adjust trans_pgd_create_copy interface

> > -int trans_pgd_create_copy(pgd_t **dst_pgdp, unsigned long start,
> > +/*
> > + * Create trans_pgd and copy entries from from_table to trans_pgd in range
> > + * [start, end)
> > + */
> > +int trans_pgd_create_copy(struct trans_pgd_info *info, pgd_t **trans_pgd,
> > +                       pgd_t *from_table, unsigned long start,
> >                         unsigned long end);
>
> This creates a copy of the linear-map. Why does it need to be told from_table?

This what done as a generic page table entries copy, but I agree, will
remove the from_table.

>
>
> > diff --git a/arch/arm64/kernel/hibernate.c b/arch/arm64/kernel/hibernate.c
> > index 8c2641a9bb09..8bb602e91065 100644
> > --- a/arch/arm64/kernel/hibernate.c
> > +++ b/arch/arm64/kernel/hibernate.c
> > @@ -323,15 +323,42 @@ int swsusp_arch_resume(void)
> >       phys_addr_t phys_hibernate_exit;
> >       void __noreturn (*hibernate_exit)(phys_addr_t, phys_addr_t, void *,
> >                                         void *, phys_addr_t, phys_addr_t);
> > +     struct trans_pgd_info trans_info = {
> > +             .trans_alloc_page       = hibernate_page_alloc,
> > +             .trans_alloc_arg        = (void *)GFP_ATOMIC,
> > +             /*
> > +              * Resume will overwrite areas that may be marked read only
> > +              * (code, rodata). Clear the RDONLY bit from the temporary
> > +              * mappings we use during restore.
> > +              */
> > +             .trans_flags            = TRANS_MKWRITE,
> > +     };
>
>
> > +     /*
> > +      * debug_pagealloc will removed the PTE_VALID bit if the page isn't in
> > +      * use by the resume kernel. It may have been in use by the original
> > +      * kernel, in which case we need to put it back in our copy to do the
> > +      * restore.
> > +      *
> > +      * Before marking this entry valid, check the pfn should be mapped.
> > +      */
> > +     if (debug_pagealloc_enabled())
> > +             trans_info.trans_flags |= (TRANS_MKVALID | TRANS_CHECKPFN);
>
> The debug_pagealloc_enabled() check should be with the code that generates a different
> entry. Whether the different entry is correct needs to be considered with
> debug_pagealloc_enabled() in mind. You are making this tricky logic less clear.
>
> There is no way the existing code invents an entry for a !pfn_valid() page. With your
> 'checkpfn' flag, this thing can. You don't need to generalise this for hypothetical users.

Ok

>
>
> If kexec needs to create mappings for bogus pages, I'd like to know why.
>

It does not.

>
> >       /*
> >        * Restoring the memory image will overwrite the ttbr1 page tables.
> >        * Create a second copy of just the linear map, and use this when
> >        * restoring.
> >        */
> > -     rc = trans_pgd_create_copy(&tmp_pg_dir, PAGE_OFFSET, 0);
> > -     if (rc)
> > +     rc = trans_pgd_create_copy(&trans_info, &tmp_pg_dir, init_mm.pgd,
> > +                                PAGE_OFFSET, 0);
>
> > +     if (rc) {
> > +             if (rc == -ENOMEM)
> > +                     pr_err("Failed to allocate memory for temporary page tables.\n");
> > +             else if (rc == -ENXIO)
> > +                     pr_err("Tried to set PTE for PFN that does not exist\n");
> >               goto out;
> > +     }
>
> If you think the distinction for this error message is useful, it would be clearer to
> change it in the current hibernate code before you move it. (_copy_pte() to return an
> error, instead of silently failing). Done here, this is unrelated noise.
>

Ok, will do that.