On Wed, 8 Nov 2023 15:45:49 +0100 Petr Tesarik <petr.tesarik1@xxxxxxxxxxxxxxxxxxx> wrote: > On 11/8/2023 3:32 PM, Halil Pasic wrote: > > On Wed, 8 Nov 2023 12:04:12 +0100 > > Petr Tesarik <petr.tesarik1@xxxxxxxxxxxxxxxxxxx> wrote: > > [..] > >>> > >>> For the sake of simplicity let us assume we only have the min_align_mask > >>> requirement. Then I believe the worst case is that we need > >>> (orig_addr & min_align_mask & PAGE_MASK) + (min_align_mask & ~PAGE_MASK) > >>> extra space to fit. > >>> > >>> Depending on how the semantics pan out one may be able to replace > >>> min_align_mask with combined_mask. > >>> > >>> Is your point that for large combined_mask values > >>> _get_free_pages(GFP_NOWAIT | __GFP_NOWARN, required_order) is not > >>> likely to complete successfully? > >> > >> Yes, that's the reason. OTOH it's probably worth a try. The point is > >> that mapping a DMA buffer is allowed to fail, so callers should be > >> prepared anyway. > >> > >> And for the case you reported initially, I don't think there is any need > >> to preserve bit 11 (0x800) from the original buffer's physical address, > >> which is enough to fix it. See also my other email earlier today. > > > > Hm. Do you mean "[PATCH 1/1] swiotlb: fix out-of-bounds TLB allocations > > with CONFIG_SWIOTLB_DYNAMIC" or a different one? > > > > I only see "[PATCH 1/1] swiotlb: fix out-of-bounds TLB allocations > > with CONFIG_SWIOTLB_DYNAMIC" but I don't think that one takes > > care of "I don't think there is any need to preserve bit 11 (0x800) > > from the original buffer's physical address". > > Yes, I mean only this patch. I want to fix memory corruption fast, while > waiting for more feedback on my understanding of the alignment masks. > What I'm trying to say is that your specific use case may not even need > a bigger allocation if the page alignment should be interpreted differently. > > Again, thank you for your in-depth inspection, because it's not totally > clear how the various alignment parameters should be interpreted. It's > difficult to write correct code then... I fully understand. Thanks for tackling this. We decided to go with a bug report and not with a fix because of the very same reasons: lack of clarity on how certain things are supposed to work. Let us see how the discussion develops. :) Regards, Halil > > Petr T
