On Wed, Dec 24, 2025 at 10:23:34AM +1300, Barry Song wrote:
> > > /*
> > > * vmap_pages_range_noflush is similar to vmap_pages_range, but does not
> > > * flush caches.
> > > @@ -658,20 +672,35 @@ int __vmap_pages_range_noflush(unsigned long addr,
> > > unsigned long end,
> > >
> > > WARN_ON(page_shift < PAGE_SHIFT);
> > >
> > > + /*
> > > + * For vmap(), users may allocate pages from high orders down to
> > > + * order 0, while always using PAGE_SHIFT as the page_shift.
> > > + * We first check whether the initial page is a compound page. If
> > > so,
> > > + * there may be an opportunity to batch multiple pages together.
> > > + */
> > > if (!IS_ENABLED(CONFIG_HAVE_ARCH_HUGE_VMALLOC) ||
> > > - page_shift == PAGE_SHIFT)
> > > + (page_shift == PAGE_SHIFT &&
> > > !PageCompound(pages[0])))
> > > return vmap_small_pages_range_noflush(addr, end, prot,
> > > pages);
> > Hm.. If first few pages are order-0 and the rest are compound
> > then we do nothing.
>
> Now the dma-buf is allocated in descending order. If page0
> is not huge, page1 will not be either. However, I agree that
> we may extend support for this case.
>
> >
> > >
> > > - for (i = 0; i < nr; i += 1U << (page_shift - PAGE_SHIFT)) {
> > > + for (i = 0; i < nr; ) {
> > > + unsigned int shift = page_shift;
> > > int err;
> > >
> > > - err = vmap_range_noflush(addr, addr + (1UL << page_shift),
> > > + /*
> > > + * For vmap() cases, page_shift is always PAGE_SHIFT, even
> > > + * if the pages are physically contiguous, they may still
> > > + * be mapped in a batch.
> > > + */
> > > + if (page_shift == PAGE_SHIFT)
> > > + shift += get_vmap_batch_order(pages, nr - i, i);
> > > + err = vmap_range_noflush(addr, addr + (1UL << shift),
> > > page_to_phys(pages[i]), prot,
> > > - page_shift);
> > > + shift);
> > > if (err)
> > > return err;
> > >
> > > - addr += 1UL << page_shift;
> > > + addr += 1UL << shift;
> > > + i += 1U << shift;
> > > }
> > >
> > > return 0;
> > >
> > > Does this look clearer?
> > >
I think so, at least the place:
<snip>
[ 2.959030] Oops: Oops: 0000 [#66] SMP NOPTI
[ 2.960004] CPU: 0 UID: 0 PID: 0 Comm: swapper/0 Not tainted 6.18.0+ #220
PREEMPT(none)
[ 2.961781] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS
1.16.3-debian-1.16.3-2 04/01/2014
[ 2.963870] BUG: unable to handle page fault for address: ffffffff3fd68118
[ 2.965383] #PF: supervisor read access in kernel mode
[ 2.966532] #PF: error_code(0x0000) - not-present page
[ 2.967682] BAD
<snip>
but it is broken for sure:
i += 1U << shift - "i" is an index in the page array.
For example if order-0 you jump 4096 indices ahead.
Should be: i += 1U << (shift - PAGE_SHIFT)
vmap_page_range() does flushing and it has instrumented KMSAN inside.
We should follow same semantic. Also it uses ioremap_max_page_shift as
maximum page shift policy.
--
Uladzislau Rezki
