Re: [PATCH v2 3/4] elf: align ET_DYN base to max folio size for PTE coalescing

[Usama Arif]

On 20/03/2026 18:58, Matthew Wilcox wrote:
> On Fri, Mar 20, 2026 at 06:58:53AM -0700, Usama Arif wrote:
>> -static unsigned long maximum_alignment(struct elf_phdr *cmds, int nr)
>> +static unsigned long maximum_alignment(struct elf_phdr *cmds, int nr,
>> +				       struct file *filp)
>>  {
>>  	unsigned long alignment = 0;
>> +	unsigned long max_folio_size = PAGE_SIZE;
>>  	int i;
>>  
>> +	if (filp && filp->f_mapping)
>> +		max_folio_size = mapping_max_folio_size(filp->f_mapping);
> 
> Under what circumstances can bprm->file be NULL?

Yeah its unnecessary here. Its used in other places and this is never
checked, so we can remove it.

> 
> Also we tend to prefer the name "file" rather than "filp" for new code
> (yes, there's a lot of old code out there).
> 

ack. will change in next revision.

>> +
>> +			/*
>> +			 * Try to align the binary to the largest folio
>> +			 * size that the page cache supports, so the
>> +			 * hardware can coalesce PTEs (e.g. arm64
>> +			 * contpte) or use PMD mappings for large folios.
>> +			 *
>> +			 * Use the largest power-of-2 that fits within
>> +			 * the segment size, capped by what the page
>> +			 * cache will allocate. Only align when the
>> +			 * segment's virtual address and file offset are
>> +			 * already aligned to the folio size, as
>> +			 * misalignment would prevent coalescing anyway.
>> +			 *
>> +			 * The segment size check avoids reducing ASLR
>> +			 * entropy for small binaries that cannot
>> +			 * benefit.
>> +			 */
>> +			if (!cmds[i].p_filesz)
>> +				continue;
>> +			size = rounddown_pow_of_two(cmds[i].p_filesz);
>> +			size = min(size, max_folio_size);
>> +			if (size > PAGE_SIZE &&
>> +			    IS_ALIGNED(cmds[i].p_vaddr, size) &&
>> +			    IS_ALIGNED(cmds[i].p_offset, size))
>> +				alignment = max(alignment, size);
> 
> Can this not all be factored out into a different function?  Also, I
> think it was done a bit better here:
> https://lore.kernel.org/linux-fsdevel/20260313005211.882831-1-r@xxxxxx/
> 
> +	if (!IS_ALIGNED(cmd->p_vaddr | cmd->p_offset, PMD_SIZE))
> +		return false;
> 


ack, will try and address this accordingly.


Thanks for the reviews!!